Okay, so my game is currently set to February 28... And Big Top just asked me to catch him a butterfly. Those don't come out until tomorrow! Is there any reason to be alarmed? I was able to catch him one just fine from the island, but I'm worried there's something wrong with my game...
In the Gamecube games insects at least would appear on the last 2 (or 3?) days before the month they are set to appear. I don't know if this applies for Wild World, City Folk, or New Leaf.
This is normal, but i'm just warning you here. Villagers will want you to catch Cicadas in June even though they don't appear until July. And they don't appear at the island either.
The previous games were pretty good in terms of villagers only asking for things available at the current time. Perhaps since butterflies can be caught on the island as noted above, these bugs can be asked for at any time of the year?
This happened to me very, very frequently in Wild World, but it hasn't happened to me in New Leaf yet. Guess it's been toned down in NL, thankfully.
Luckily you can just turn down the request or ignore it until the next day. Sucks to have to do that, but since you don't have the option to formally drop a villager's ~quest~ after you pick it up... :/