Tuesday evening/Wednesday in general, but I've seen drops on Thursdays and Fridays too. Weekends are not the time to buy, but buying airfare is always a gamble. I always choose Jetblue since I can get a credit if the fare drops. I had a drop this week for my trip in 2 weeks and our family ended up with an $80 credit. Our return flight dropped to $69! I didn't see that coming at all. I use Yapta to help track fare drops for flights I've purchased and flights I hope to purchase.