The word familial has to do with all things relating to family. A familial gathering is one in which family has come together. A familial bond is a strong connection between family members.
It's easy to spot the Latin root familia meaning "family" in the word familial. This is a word that describes events, relationships, or all other things having to do with family. When you talk about a familial bond, you are talking about the kind of intimate connection that only family members can share. If you are interested in genealogy, you have an interest in familial history.