Disable merging if merge day is in the future

app/helpers/merge_requests_helper.rb

@@ -276,4 +276,8 @@ module MergeRequestsHelper
   def any_organisations_share_logs?(organisations, type)
     organisations.any? { |organisation| organisation.send("#{type}_logs").filter_by_managing_organisation(organisations.where.not(id: organisation.id)).exists? }
   end
+
+  def begin_merge_disabled?(merge_request)
+    merge_request.status != "ready_to_merge" || merge_request.merge_date.future?
+  end
 end

app/views/merge_requests/_notification_banners.html.erb

@@ -19,3 +19,11 @@
     No changes have been made. Try beginning the merge again.
   <% end %>
 <% end %>
+
+<% if @merge_request.merge_date&.future? %>
+  <%= govuk_notification_banner(title_text: "Important") do %>
+    <p class="govuk-notification-banner__heading govuk-!-width-full" style="max-width: fit-content">
+      This merge is happening in the future. Wait until the merge date to begin this merge.
+    </p>
+  <% end %>
+<% end %>

app/views/merge_requests/show.html.erb

@@ -12,7 +12,7 @@
 </h1>
 <% unless @merge_request.status == "request_merged" || @merge_request.status == "processing" %>
 <div class="govuk-button-group">
-  <%= govuk_button_link_to "Begin merge", merge_start_confirmation_merge_request_path(@merge_request), disabled: @merge_request.status != "ready_to_merge" %>
+  <%= govuk_button_link_to "Begin merge", merge_start_confirmation_merge_request_path(@merge_request), disabled: begin_merge_disabled?(@merge_request) %>
   <%= govuk_button_link_to "Delete merge request", delete_confirmation_merge_request_path(@merge_request), warning: true %>
 </div>
 <% end %>