Many online communities try to celebrate important days.
It might be international women's day, MLK day, veterans day or their own birthday.
But online celebrations are a disappointing experience.
Most members aren't together at the same time. They find it hard to be in the spirit of the day. At best you get a few blog posts, a few congratulations, and then the day has passed.
I suggest you pick one day per year that really matters to your community and build up to it.
Plan the release of something exclusive to members. Try to meet in person if you can. Get the key people to release exclusive information. Do a countdown. Have a goal you're trying to hit during that day. It might be 17,000 tweets, 50 blog posts, or anything that has an impact that extents beyond the community.
We can do celebration days much better.