On the 20th of every month, the FEC releases campaign finance reports for federal candidates. If you have ever gotten an email from a candidate talking about "FEC deadlines", this is what they are talking about. There is no real "deadline", though, just a reporting boundary. It's importance to the campaigns is more that if you can get enough people and money through the door just before the cutoff, you look better and can use it for the next round of fundraising and media tomfoolery.