These are the main bank holidays and celebration days for UK in 2023, occasions you may want a card or gift for. Source on the site you can view holidays for other countries and religions.

1 Jan New Year’s Day
2 Jan ‘New Year’s Day’ observed
3 Jan 2nd January (substitute day) (Scotland)
5 Jan Twelfth Night

14 Feb Valentine’s Day
21 Feb Carnival / Shrove Tuesday / Pancake Day

17 Mar St Patrick’s Day (Northern Ireland)
19 Mar Mother’s Day

7 Apr Good Friday
9 Apr Easter Sunday
10 Apr Easter Monday (ENG, NIR, WAL)
23 Apr St. George’s Day

1 May Early May Bank Holiday
6 May The Coronation of King Charles III
8 May Bank Holiday for the Coronation of King Charles III
29 May Spring Bank Holiday

17 Jun King’s Birthday
18 Jun Father’s Day

12 Jul Battle of the Boyne (Northern Ireland)

7 Aug Summer Bank Holiday (Scotland)
28 Aug Summer Bank Holiday (ENG, NIR, WAL)

31 Oct Halloween

5 Nov Guy Fawkes Day
12 Nov Remembrance Sunday
30 Nov St Andrew’s Day (Scotland)

24 Dec Christmas Eve
25 Dec Christmas Day
26 Dec Boxing Day
31 Dec New Year’s Eve

Another great site for holidays and more unusual “celebrations” is National Today and it include awareness days too.