Christmas Day shifted onto Boxing Day (UK)

[koterpillar]

In 2022, Christmas Day (Dec 25) is a Sunday. At least in the UK, the substitute holiday is not Dec 26 as that already is a holiday (Boxing Day) but Dec 27; however, desktopCal.js displays both on Dec 26.

I intend to make a PR with Australian holidays (with the same logic).

Would you be interested in a PR that changes/adds the holiday observation logic as follows:

• Whenever a holiday falls on a weekend, the observation date is moved to the next non-holiday
• Original holiday is marked on the weekend
• A copy with "(observed)" is created on the extra day off