As this year is a Judaic leap year, we shall be observing Purim Kattan ("little" Purim) on the 14 and 15 of Adar I (Feb. 24 and 25 on the Gregorian calendar.) (This holiday occurs every Judaic leap year, not just this one, of course.)
I vaguely recall a discussion in Talmud Megilla that explained that the reason the "real" Purim is moved to the 2nd Adar rather than the 1st is to make the miracle of Purim occur as close as possible to the miracle of Passover.
From further recollection, Shulchan Aruch states that on Purim Kattan, we do not recite Tachanun in the prayers, and there is a "suggestion" to make some type of small festive meal. (I don't know how many people actually make such a meal.)
I'm unaware of the Talmud or any other source mentioning giving any importance to Purim Kattan, and the Shulchan Aruch also doesn't seem to give a reason or source for it other than stating what to do on that day.
Why is it given any importance, and what is the earliest known source that mentions it?