Just like different countries and cultures have different holidays, so does your community. Or it can, at least.
It’s common for a country to celebrate the birth of their nation. The day that your community launched can be a holiday. As can the half-birthday, if you so choose.
If you have any sort of annual program, the start of that program is a type of holiday. For example, a yearly awards event where members nominate and vote for their favorites.
They don’t all have to be so serious or structured. Remember that day when Christopher Walken registered and posted on your forums? That shall now be known as Christopher Walken Day on your community.
There is a lot of room for creativity, but just like holidays bring people together throughout the world, they can do the same for your members. Create a list of the community holidays that you celebrate, and post it where everyone can see. And don’t forget to celebrate them!