Where I’m at in the USA, tomorrow is the start of daylight saving time.
The IANA (Internet Assigned Numbers Authority) maintains a database of time zone information. This includes data on when times zones implement daylight saving time, for both past and future years.
The data is available for download here.

Links:

I’ve blogged here on time zones, mostly on using SQL Server to convert between time zones:
Convert String To DateTime For A Specific Time Zone

An interesting post on time zones:
Ulysse Carion – Australia/Lord_Howe is the weirdest timezone

Wikipedia – Daylight Saving Time

Edit: May 17, 2025
I wanted to add a link to an interesting article on UTC:
Zach Holman – UTC is enough for everyone …right?