Only show tinsel on Christmas Day, Boxing Day & New Year's Day bank h…

…olidays

- Previously, we showed tinsel on all bank holidays that fell in
  December and January, assuming that they were only Christmas Day,
  Boxing Day and New Year's Day.
- This year (and presumably other years in the past/future), St Andrew's
  Day in Scotland was a substitute day on 2nd December. We showed
  tinsel, when we should have just showed normal bunting.
- This also accounts for all possible substitute Christmas/New Year bank
  holidays.

app/models/calendar.rb

@@ -47,7 +47,7 @@ def show_bunting?
   end
 
   def bunting_styles
-    if [1, 12].include? Time.zone.today.month
+    if christmas? || new_year?
       "tinsel"
     else
       ""
@@ -67,4 +67,12 @@ def content_id
   def body
     @data["body"]
   end
+
+  def christmas?
+    Time.zone.today.month == 12 && (25..28).include?(Time.zone.today.day)
+  end
+
+  def new_year?
+    Time.zone.today.month == 1 && (1..4).include?(Time.zone.today.day)
+  end
 end