Or, more specifically, what is/are the reason(s) for not saying tachanun on various occasions and how does Tu b'Shvat fit in. (I'm looking for classic reasons, not 'it's got to be somebody's birthday ...
Where did the idea of doing a Seder on Tu b'Shvat come from? Why a Seder, not just the eating of fruit? Why do we call it a Seder if there is no formalized way of doing it?