Yesterday we wrote about a one-day JetBlue fall sale with one-way fares as low as $39 from Boston for travel through Dec. 15. The carrier has extended that sale until midnight tonight. No wonder. That decision comes after Southwest and American also launched sales of their own that run until midnight tomorrow night for travel through Dec. 14. The sales all come with restrictions (some have 10-day advance purchase restrictions) and all have blackout dates and limits on travel days. But it looks like the best and widest range of discounts and destinations are available though JetBlue and American. Here is the fine print for JetBlue, American, and Southwest. Start shopping.