I fly JFK-LAS a bit, and I've found that a few months out from departure, it's unlikely that DL will lower their fares, and if they do, it won't be by much. But I have seen the opposite, with DL raising their fares quite a bit.

Fares to LAS for the weekend you mention are quite a bit higher than the weekends around them. I was also planning on going that weekend, but moved it to a later weekend due to the pricing.