Most, if not all, societies have their own holidays and rituals - they are part of what defines a group. When dealing with foreign or alien cultures in writing, one way to highlight some of values of the group is to describe and show a holiday celebration, ceremony, or ritual. Depending on how it's done, it can either show how similar they are to what the reader knows (birthdays/namedays, harvest festival, etc) or how different they are.
For this exercise, describe a holiday/ceremony/ritual of a foreign or alien culture. This may be done in one of two ways. The first would be in a factual, structured description, similar to an encyclopedia entry or college essay. It is often helpful in writing to have highly detailed background information that you can draw upon in your writing - even if you don't use/show/tell it all, you'll still convey a feeling that you know what you're talking about if you've done the "research" and fully developed what you need, rather than winging it and writing off the fly. The second method is to write a scene that might appear in a story in which the characters either observe or take part in the holiday celebration/ritual. Of course, this is pretty much "writing off the fly," but it should be noted that there's nothing wrong with that, either. Some people do better with one style than the other.
Granted, one could write both, developing the history and methods of the holiday in expository prose before writing a narrative and dialog, or it could be that the narrative comes first, and the exposition is developed afterward.