Something weird happens to FMC on her birthday. When she turns 18/21 (?) she finds a time capsule (or something) buried by her mother that is filled with all sorts of magic things passed down from generation to generation. Finds out that there are others she was supposed to meet up with. They were all born on same day/same time and are part of a major fight for survival against the ultimate bad.
I picture her as a witch. Not sure if the “others” are all witches, too, or if they have their own story as a separate entity (warlock, shape-shifter, vampire, etc..)