Easter

It involves a good amount of supposition to understand what might have "really happened," but is important to get a sense of the context of the times. This is so even if you are an atheist and don't accept the (rather hard to take seriously, if you think too much about it) supernatural components. There is also a way to accept a meaning to it all, like Christmas, it having certain somewhat universal aspects such as rebirth. Ditto Passover in some sense.