Add Christmas dispatch dates

app/templates/views/guidance/using-notify/delivery-times.html

@@ -31,6 +31,11 @@ <h2 id="text-messages"  class="heading-medium">Text messages</h2>
 
   <h2 id="letters" class="heading-medium">Letters</h2>
   <p class="govuk-body">Letters are printed at 5:30pm and dispatched the next working day (Monday to Friday).</p>
+
+  <div class="govuk-inset-text">
+    Letters sent after 2pm on 24 December will be dispatched on Monday 30 December.
+  </div>
+
   <p class="govuk-body">First class letters are delivered one day after they’re dispatched. Second class letters are delivered 2 days after they’re dispatched. Royal Mail delivers from Monday to Saturday, excluding bank holidays.</p>
   <p class="govuk-body">Letters to Europe are delivered 3 to 5 days after they’re dispatched. Letters sent anywhere else in the world take 5 to 7 days to arrive.</p>