One I did to someone else, the other was done to me...
One year, many years ago, the start of Daylight Savings Time began on April 1st. Before going to bed, my husband and I went through the house changing all the clocks. I waited for him to fall asleep and I pushed them all ahead an extra hour. He woke up that Sunday morning and rushed off to work, without ever realizing what I had done till he arrived at work and went to change the big clock on the wall.
Now what made this one of the best pranks ever is how for 3 days of every year, whenever Daylight Savings Time begins or ends, and on April 1, my husband goes into a time panic, thinking I am going to prank him like that again. He becomes such a wreck and I just laugh. I never have to prank him again. That one time was good enough to affect him for the rest of his life.
Now the one that got me wasn't really a prank at all. When I was back in high school, I received a phone call very early in the morning on April 1, from someone that was trying to tell me that school was canceled because of snow. Even though I was half asleep, I was quite aware of the date, and wished her a Happy April Fool's Day before I hung up. I ended up getting ready for school and going downstairs to leave, opening the front door to a vision of about 2 feet of snow. Yeah, the joke was on me, that phone call was for real.