Holidays for the whole of Britain:
New Year's Day 1 January
Easter March/April (Good Friday to Easter Monday inclusive)
May Day First Monday in May
Spring Bank Holiday Last Monday in May
Summer Bank Holiday Last Monday in August
Christmas Day 25 December
Boxing Day 26 December
If a public holiday falls on a weekend, the nearest Monday is usually taken instead. Most businesses and banks close on official public holidays (hence the quaint term 'bank holiday').
On public holidays, some small museums and places of interest close, but larger attractions have their busiest times. If a place closes on Sunday, it'll probably be shut on bank holidays as well.
Virtually everything – attractions, shops, banks, offices – closes on Christmas Day, although pubs are open at lunchtime. There's usually no public transport on Christmas Day, and a very minimal service on Boxing Day.
Roads get busy and hotel prices go up during school holidays. Exact dates vary from year to year and region to region, but are roughly as follows:
Easter Holiday Week before and week after Easter.
Summer Holiday Third week of July to first week of September.
Christmas Holiday Mid-December to first week of January.
There are also three, week-long 'half-term' school holidays – usually late February (or early March), late May and late October. These are staggered, so the whole country is not on holiday during the same week.