If there's a promo on now and it states the promo runs until after the dates you're planning on flying, then they won't pull the plug early on the promo. Of course they might sell out all the promo seats for the days you want, so you run that risk by waiting. By waiting, you might get lucky and TG might add a new and better promo.

If the price is acceptable, I'd personally book the ticket now if you're sure of your travel plans. If not sure, I'd wait.