Today, my family celebrates a birthday.
Every year, each person is celebrated on their own day. On that day, the person is treated a little nicer by everyone. There might be gifts, a feast or a party. Visitors might gather from all around, including distant places. And there are likely to be stories shared about the celebrated person and birthdays past.
Birthdays could be excellent inspiration for game features, particularly in MMOs and other online multiplayer games. Each individual player has their own day, and on that day the player is given more than a congratulatory message.
In an item-centric game, perhaps the player can pull something from a grabbag or select from a variety of free items.
Or the player might be granted one day's access to a special play-scenario (the access could be saved for another day).
Or the player's stats might be boosted for a day. Experience points might be earned quicker, health/mana/etc refreshed quicker.
There are a thousand possibilities. The gist is that every player will have one day during which they will be appreciated as an individual and reminded that he or she is important. If you can get fellow players involved, great, but the feature should be able to stand on its own. No matter what schedules are like, no matter where everyone is in the game or what they have at their disposal, the celebrated player will experience a special and memorable day.