diff --git a/_config.yml b/_config.yml
index 772a75c..ca4ac35 100644
--- a/_config.yml
+++ b/_config.yml
@@ -35,16 +35,18 @@ course_email: COURSE_EMAIL@berkeley.edu
 gradescope_course_id: 123456 # you can find this in the Gradescope URL after /courses
 bcourses_course_id: 123456 # Same as above, but for bCourses. Leave blank if not in use...
 ed_course_id: 123456 # Again, same as above.
-sememster: spYY | suYY | faYY # set for the current seemester
+semester: spYY | suYY | faYY # set for the current semester
+semester_start_date: '2025-01-21' # Required for the archive banner logic (YYYY-MM-DD)
+semester_end_date: '2025-05-09' # Required for the archive banner logic (YYYY-MM-DD)
 # TODO(setup): Set this to the file path of your course logo image, or leave it blank to display your course name.
 # To edit the size and positioning of the logo/course name, see _sass/custom/course_overrides.scss
 logo: /assets/images/berkeley_walking_bear.png
 # This should be one of eecs, dsus, stat
 # (Future) This will control some footer text, and later custom styling.
 course_department: dsus
-# This should be the page of all class archives
-# Typically just / for DS courses (with a visible index page), or /archives if you're hosting your own, or a link to the inst.eecs page
-# If you have no archive page, comment this line out or leave blank.
+# This path/URL is used by the archive banner shown when the site date is outside the semester window.
+# Keep this key present. Default `/` is fine for class-listing homepages, and `/archive` is also a good option.
+# We recommend listing archived offerings as "Archive" or "Not Updated" in course listings.
 class_archive_path: /
 
 # TODO(setup): Remove this line if your favicon is named favicon.ico and is in the root directory,
diff --git a/_includes/header_custom.html b/_includes/header_custom.html
new file mode 100644
index 0000000..f11b25c
--- /dev/null
+++ b/_includes/header_custom.html
@@ -0,0 +1,41 @@
+{% assign archive_url = site.class_archive_path | absolute_url %}
+<div id="archive-banner" class="archive-banner" role="status">
+  This site is archived. Please check:
+  <a id="archive-banner-link" class="archive-banner-link" href="{{ archive_url }}">{{ archive_url }}</a>
+</div>
+
+<script>
+  (() => {
+    const semesterStart = {{ site.semester_start_date | jsonify }};
+    const semesterEnd = {{ site.semester_end_date | jsonify }};
+    if (!semesterStart || !semesterEnd) {
+      return;
+    }
+
+    const parseDate = (value) => {
+      const parsed = new Date(value);
+      return Number.isNaN(parsed.getTime()) ? null : parsed;
+    };
+
+    const startDate = parseDate(semesterStart);
+    const endDate = parseDate(semesterEnd);
+    if (!startDate || !endDate) {
+      return;
+    }
+
+    endDate.setHours(23, 59, 59, 999);
+    const currentDate = new Date();
+    const isOutsideSemester = currentDate < startDate || currentDate > endDate;
+    if (!isOutsideSemester) {
+      return;
+    }
+
+    const banner = document.getElementById('archive-banner');
+    if (!banner) {
+      return;
+    }
+
+    banner.style.display = 'block';
+    document.body.classList.add('archive-banner-visible');
+  })();
+</script>
diff --git a/_plugins/config_validator.rb b/_plugins/config_validator.rb
index d49aa59..8d845f3 100644
--- a/_plugins/config_validator.rb
+++ b/_plugins/config_validator.rb
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require 'date'
+require 'uri'
+
 # A UC Berkeley-specific validator for Jekyll sites
 # This class validates the following options:
 # 1) Ensures required attributes are present in the config file.
@@ -44,7 +47,10 @@ class ConfigValidator
       url: :validate_clean_url,
       baseurl: :validate_semester_format,
       course_department: :inclusion_validator,
-      color_scheme: :inclusion_validator
+      color_scheme: :inclusion_validator,
+      semester_start_date: :validate_iso8601_date,
+      semester_end_date: :validate_iso8601_date,
+      class_archive_path: :validate_archive_path
     }.freeze
 
     attr_accessor :config, :errors
@@ -61,13 +67,15 @@ def validate
         send(validator, key, config[key.to_s]) if @config.key?(key.to_s)
       end
 
+      validate_semester_date_range
+
       raise ConfigValidationError, errors if errors.length.positive?
 
       puts 'Passed Berkeley YAML Config Validations'
     end
 
     def validate_keys!
-      required_keys = %i[baseurl course_department]
+      required_keys = %i[baseurl course_department semester_start_date semester_end_date class_archive_path]
       required_keys.each do |key|
         errors << "#{key} is missing from site config" unless @config.key?(key.to_s)
       end
@@ -95,6 +103,40 @@ def inclusion_validator(key, value)
       allowed = self.class.const_get("VALID_#{key.upcase}")
       errors << "`#{key}` must be one of #{allowed} (not '#{value}')" unless allowed.include?(value)
     end
+
+    def validate_iso8601_date(key, value)
+      return if parse_iso8601_date(value)
+
+      errors << "`#{key}` must be a valid ISO-8601 date string (YYYY-MM-DD), not '#{value}'"
+    end
+
+    def validate_semester_date_range
+      start_date = parse_iso8601_date(config['semester_start_date'])
+      end_date = parse_iso8601_date(config['semester_end_date'])
+      return unless start_date && end_date
+      return unless start_date > end_date
+
+      errors << '`semester_start_date` must be on or before `semester_end_date`'
+    end
+
+    def validate_archive_path(_key, value)
+      path = value.to_s.strip
+      return errors << '`class_archive_path` cannot be blank' if path.empty?
+      return if path.match?(%r{^/})
+
+      uri = URI.parse(path)
+      return if uri.is_a?(URI::HTTP) && uri.host
+
+      errors << "`class_archive_path` must be an absolute path starting with `/` or a full URL, not '#{value}'"
+    rescue URI::InvalidURIError
+      errors << "`class_archive_path` must be an absolute path starting with `/` or a full URL, not '#{value}'"
+    end
+
+    def parse_iso8601_date(value)
+      Date.iso8601(value.to_s)
+    rescue Date::Error
+      nil
+    end
   end
 end
 
diff --git a/_sass/berkeley/variables.scss b/_sass/berkeley/variables.scss
index 366f259..828c41b 100644
--- a/_sass/berkeley/variables.scss
+++ b/_sass/berkeley/variables.scss
@@ -12,6 +12,7 @@ $serif-font-family: 'Source Serif 4', 'Georiga', 'Baskerville', serif;
 
 $berkeley-blue-old: #003262;
 $berkeley-blue: #002676; // rgb(0, 38, 118);
+$berkeley-rose-medium: #d02670;
 
 // Colors
 $link-color: $berkeley-blue;
diff --git a/_sass/custom/custom.scss b/_sass/custom/custom.scss
index 50df695..a9d693e 100644
--- a/_sass/custom/custom.scss
+++ b/_sass/custom/custom.scss
@@ -137,9 +137,34 @@ p.note::before {
     @include btn-color($white, #6929c4);
   }
   
-  .btn-officehours{
-    @include btn-color($white, #009d9a);
-  }
+.btn-officehours{
+  @include btn-color($white, #009d9a);
+}
+
+.archive-banner {
+  display: none;
+  position: fixed;
+  top: 0;
+  right: 0;
+  left: 0;
+  z-index: 999;
+  padding: 0.75rem 1rem;
+  background-color: $berkeley-rose-medium;
+  color: #fff;
+  font-weight: 700;
+  text-align: center;
+}
+
+.archive-banner-link,
+.archive-banner-link:visited {
+  color: #fff;
+  font-weight: 700;
+  text-decoration: underline;
+}
+
+body.archive-banner-visible {
+  padding-top: 3rem;
+}
 
 @if $color-scheme == dark {
   @include dark-mode-overrides;
diff --git a/spec/jekyll/config_validator_spec.rb b/spec/jekyll/config_validator_spec.rb
new file mode 100644
index 0000000..b617bf3
--- /dev/null
+++ b/spec/jekyll/config_validator_spec.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_relative '../../_plugins/config_validator'
+
+RSpec.describe Jekyll::ConfigValidator do
+  let(:base_config) do
+    {
+      'baseurl' => '/sp26',
+      'course_department' => 'dsus',
+      'semester_start_date' => '2026-01-21',
+      'semester_end_date' => '2026-05-09',
+      'class_archive_path' => '/'
+    }
+  end
+
+  it 'validates with required semester dates present' do
+    validator = described_class.new(base_config)
+
+    expect { validator.validate }.not_to raise_error
+  end
+
+  it 'requires semester_start_date and semester_end_date' do
+    config = base_config.except('semester_start_date', 'semester_end_date')
+    validator = described_class.new(config)
+
+    expect { validator.validate }
+      .to raise_error(ConfigValidationError, /semester_start_date is missing.*semester_end_date is missing/m)
+  end
+
+  it 'requires class_archive_path' do
+    validator = described_class.new(base_config.except('class_archive_path'))
+
+    expect { validator.validate }
+      .to raise_error(ConfigValidationError, /class_archive_path is missing from site config/)
+  end
+
+  it 'allows class_archive_path as a full URL' do
+    validator = described_class.new(base_config.merge('class_archive_path' => 'https://c88c.org/archive'))
+
+    expect { validator.validate }.not_to raise_error
+  end
+
+  it 'rejects class_archive_path values that are not paths or full URLs' do
+    validator = described_class.new(base_config.merge('class_archive_path' => 'archive'))
+    expected = %r{`class_archive_path` must be an absolute path starting with `/` or a full URL}
+    expect { validator.validate }.to raise_error(ConfigValidationError, expected)
+  end
+
+  it 'validates ISO-8601 semester dates' do
+    validator = described_class.new(base_config.merge('semester_start_date' => 'spring-2026'))
+
+    expect { validator.validate }
+      .to raise_error(ConfigValidationError, /`semester_start_date` must be a valid ISO-8601 date string/)
+  end
+
+  it 'requires the start date to be on or before the end date' do
+    validator = described_class.new(base_config.merge('semester_start_date' => '2026-06-01'))
+
+    expect { validator.validate }
+      .to raise_error(ConfigValidationError, /`semester_start_date` must be on or before `semester_end_date`/)
+  end
+end