Birthday parties are ways to celebrate someone on the day that they were born. If a person's birthday happens to be April Fool's Day, or April 1st in the United States, their birthday has probably always been marked by practical jokes and tricks. Even someone with an early April birthday or a late March birthday will enjoy a fun filled April Fool's Day birthday party.
April Fool's Day is a day full of jokes. So, throw the person a fake party. You can create invitations and decorations to a party that doesn't exist, and invite them to it on April Fool's Day. When they show up, the person will believe that no one has come to their party. Then, you can surprise them with another party in a different location. Throwing a fake party can be considered a mean trick, so be sure that you have another, better party waiting for them.
Play practical jokes on the person at their birthday party, and on the guests. Or, have each guest bring a practical joke to play on the entire party. Some practical jokes can make a mess, so be sure that you have told everyone at the party what you are planning so it is expected.
April Fool's Day is full of surprises already, so this can be the best time to throw someone a surprise birthday party. Plan the party just as you normally would, except be sure that you don't tell the birthday guy or gal about the party. Then, get them to the location under a different pretense and surprise them when they walk in the door. This can be the ultimate April Fool's Day joke to play on someone.
If playing jokes on someone isn't what you are looking for, April Fool's Day can also be about the celebration of the impending Springtime. Use Spring as a theme instead of April Fool's Day. Have plenty of flowers and pastel colors and decorate using items like flower buds, birds, bugs, and sunny skies. Sometimes a Springtime theme can be a welcome change for someone, especially if they have always had traditional April Fool's Day birthday celebrations.