Plum Benefits emailed me about them, but that date/times are not included. They often put out promo codes for their own box office that are closer to the dates of the show, but the key is the flyer says only on "eligeable dates/times/seats" so you may show up and find nothing applies that day for thier own code as well. The best I have gotten with their direct offer was BOGO but it was always on a weekday at like a 2pm or 7pm show and was orchestra seats (what was left in middle or towards back).
If it is the first Saturday in December in the middle of the day you are at peak time, especially as tour busses come in for day trips, so discounts may be few and far between as some discount sites show that it is a blackout date. I would instead look for economical seat choices. I think 3rd Mezz is way too high up to enjoy (did that once last minute, terrible choice). 2nd or 1st Mezz if you are in the first 5 rows towards center are not bad (2nd over third but it will cost more- may cost as much as orchestra seats). Orchestra is very big, if you do choose lower price seats there try to be as far center as you can towards the back for better viewing of dance formations, although you may have a harder time seeing things on the balconies (which to me those actors/actresses are secondary to the rockettes). Also mezz may be better these days as they do things up towards the ceilings thanks to technology so with seats at back of orchestra your view may be obstructed.
