Valentines in April is a classic trick used to lure an unsuspecting single male to a faltering party or social gathering. Typically, the host will specify that there are attractive single women at said party. Alternately, the hostess may invite you to go out with her and her cute girlfriends.
At any rate, the punch line is when you walk in and are inevitably the only person without a date, leading to conflicts over women with boyfriends and/or fuckbuddies. This trick can be used for a variety of purposes including but not limited to

A) The amusement of the host and his guests at your singledom,
B) To deceive someone funny or interesting into attending so the evening isnt dominated by gushy, boring couplespeak, or
C) To increase the amount of sausage at a party hosted by a presumeably unattractive single woman.