To understand why this is funny--you have to know the back story.
Right before AFD in our area there was a $150 million dollar lottery winning that was unclaimed.
Additionally, my parents who had been married for 30 years and divorcing for the last 15 years had just finalized their divorce 6 months earlier. Both of my parents have a very good sense of humor--especially relating to how long it took them to divorce (my personal favorite were the legal discussions about who got their Wok, it was a wedding gift and had never been out of the box---one day since they had both complained to me about it, and neither one wanted it, I went to the house took the Wok and called both lawyers and told them the Wok was off the table and now at the local goodwill). They even lived in the same house for 13 of the 15 years they were divorcing, and not due to lack of money.
So here is the prank: I talked to my mom in the morning and told her not to tell anyone but that my dad had the winning ticket--he was know to drop a couple bucks at the gas station when the jackpots got really large. Now she complained, about how it was just her luck she'd finally get the divorce and he wins "the Big One". I told her a couple of times that he wasn't ready for the information to be public, he was trying to put together a "winning tax plan", etc. Now I didn't think much of it and went to work.
When I get home from work there were 20 messages on my machine, my mom's best friend, my uncle (mom's brother), some cousins (including one who realized it was April 1), all talking about the lottery win. [My original plan was not to have her tell anyone, but to just tell her later in the day]. So I called my mother and asked her if she wanted to come over for dinner. She makes some pithy comment, how she better come for a free meal since "(insert derogatory word here) won all the money" a very out of character comment.
While cooking I asked her to help me do some birthday party planning and hand her a calendar opened to April. Next I ask if she has told anyone about our little "secret". She turns beet red, and inquires why I am asking--I tell her about the full voicemail box. Then I tell her to make a list of all the people she has called and shared the "secret". My mother is not a big talker, but that day she called 27 people including their mutual dentist. At this point, I am about to fall out of my chair---and my phone is still ringing while she is writing her list.
Finally I asked her if she knew what day it was.....she looks at the calendar and exclaims in only the way someone who has just been pranked can exclaim "oh sh*t". The look on her face was priceless.

I got her a wine glass, a bottle of wine, and the phone and told her it was time to call everyone and let them in on my prank.
Seriously she was on the phone laughing for 5 hours with all her friends, she got pretty smashed, and we had to drive her home. The best part is she has played this same trick on others--who were not called when she was the victim.
We still laugh about this 10 years later--even my Dad thinks it is funny--and he got phone calls from people asking for loans for months. At one point his answering machine said, "Hello, you have reached ---- and I am unable to answer the phone. Additionally, if you are requesting a loan I have not recently won the lottery." He said he heard from a lot of "old friends"--amazing how quickly Mom's 27 conversations multiplied.