Planes around Christmas are full and they don't need to offer a single seat on a Lite fare to achieve that. There will be no Lite fare today and no Lite fare tomorrow if you are flying at the busiest time of the year. For all other periods, there will be at least a couple of seats released on Lite fares. Most often immediately, sometimes later during a targeted sale, sometimes when revenue management feels that sales are behind target.

If you plan to travel during Christmas, see whether you can snatch a Premium Economy redemption ticket. There should be some seats released when the schedule opens up.