I’ll say this, purely from a reasoning standpoint and not from a research stand point. With my wife, we dated for a year, cohabitated for a year, and were engaged for a year before we were married. This made sense to me because you get to experience all the holidays at every point in your life, and it let’s you see each other how you exist throughout different points in the year. If you need the AC cranked in the summer, and your parent can’t stand that, or if you want to bake all winter and stay inside while your partner wants to go out all the time, you aren’t able to discover that without cohabitating for at least 1 year.