Restrict calendar picker to Sundays only

sspromotions/jinja2/sspromotions/bulletin_edit.html

@@ -41,7 +41,7 @@
 						<input class="twelve wide column" type="text" name="date" value="{{ item.date or '' }}">
 					</div>
 				</div>
-				<div style="margin-top: 1.5em;">The item will be scheduled for the first bulletin post on or after this date.</div>
+				<div style="margin-top: 1.5em;">Choose the date to publish the bulletin item (must be a Sunday).</div>
 			</div>
 		</div>
 		<div class="ui required inline grid field">
@@ -110,6 +110,9 @@
 				date: function(date, settings) {
 					return date.getFullYear() + '-' + leftpad(date.getMonth() + 1) + '-' + leftpad(date.getDate());
 				}
+			},
+			isDisabled: function(date, mode) {
+				return date.getDay() !== 0; // Sunday only
 			}
 		});