It depends on where you live.
If you live anywhere in the EU, then it is (strictly speaking) 0100 GMT on Sunday 25 March. So Brits and Portuguese put theirs forward at 1 am, French and Germans put theirs forward at 2 am and Finns and Greeks at 3 am.
Most people put them forward either just before bed or first thing on Sunday morning.
Just don't turn up at church an hour late! Priests and ministers have been known to take a dim view of this.
In USA/Canada it used to be first Sunday in April (April 1 this year) but I believe it's been changed this year. US or Canadian ABers might like to set us right on this one.