It is my observation that a large number of people are affiliated to various religions for the primary reason of the sense of community, gathering and ceremonies they get by going to churches, mosques and temples.

We are indeed social animals, and we crave for such kind of social activities.
For many it would be dreadful to leave religion, for it would constitute loosing these gatherings, ceremonies, festivals and holidays.
Atheists on the other hand have seldom such opportunities for social activities. There is a rather dangerous image of atheists going around in general that - all an atheist does on a Sunday is sit in front of his desk and blog.

It would be great for atheists and rationalists, to recognize the virtue in such social activities, and to provide with ideas for rational, scientific and, by definition, much better alternatives - alternatives for Sunday churches and other gatherings.

I think atheist communities in cities would do good to organize gatherings such as -
Organize talks and debates by notable personalities, organize scientific presentations on topics like astronomy, biology, physics, etc which would be greatly educational and valuable; organize physical and healthy activities like marathons, biking tours, hiking, etc; organize art exhibitions and competitions, organize social activities like clean-up-the-city, planting trees, cleaning a lake, etc.

Such activities will not only fill the gap and provide a platform for social gathering for people, but also further the scientific and social causes, and increase the image of atheists in general, would  probably also lead to raise some fund that could be used in positive ways.