Below are some non-date-specific holidays and other events. Add Not Date-Specific Holidays has tips for adding them to Datebk.
Daylight Savings Time - start: last Sunday in March
Mother's Day: 2nd Sunday in May (most of Europe)
Daylight Savings Time - end: last Sunday in October
New Year's Day: 1 January, unless that falls on a weekend, in which case 1st Monday in January
New Year Holiday: day after New Year's Day (Scotland only)
Mothering Sunday: 4th Sunday of Lent (Lent starts 40 days before Good Friday)
May Bank Holiday: 1st Monday in May
Spring Bank Holiday: last Monday in May
Late Summer Holiday: 1st Monday in August (Scotland only)
Late Summer Holiday: last Monday in August (not Scotland)
Martin Luther King Day: 3rd Monday in January
President's Day: 3rd Monday in February
Mother's Day: 2nd Sunday in May
Memorial Day (observed): last Monday in May
Father's Day: 3rd Sunday in June
Labor Day: 1st Monday in September
Columbus-Indigenous People's Day (observed): 2nd Monday in October
Daylight Savings Time ends: last Sunday in October
Election Day: 1st Tuesday after 1st Monday in November
Thanksgiving: 4th Thursday in November