Hi thenextbillgates,
Most of the better manufacturers or retailers don’t have or need special sales to be a good quality/value choice and they sell their mattresses at fair prices every day of the year so there really is no best time to buy from either Select Foam or Tempflow.
Just for reference … some of the better online memory foam options I’m aware of are also listed here (including a link to a caution about Select Foam).
Phoenix