Many parking restrictions don’t run on Sundays, and some authorities don’t charge or enforce on bank holidays – but neither of these are things you can take for granted.
Where the signs don’t say which days they apply on, they apply every day, Sundays and bank holidays included. Unless they specifically say “except bank holidays”, they apply on bank holidays, too. Some restrictions, particularly in areas that get a lot of tourists, may even apply only on bank holidays. Always check the signs, and if unsure, double check.
See also reading difficult signs.