They always try to open the ski station before the national holidays at the start of December.

The dates of the national holidays are:

6th December:  dia de la constitución  (day of the Spanish constitution)
8th December: dia de la inmaculada (day of the immaculate conception)

If one of the public holidays falls on a Sunday then the day off is passes to the Monday.

As there are 2 public holidays many people people join them together and this provides a 4 or even 5 day holiday. 

If you have flexibility of dates it is best to avoid going to the ski station on these holidays because it is very full and the ski lifts have long queues.

I have just looked at the Ideal Newspaper and the ski station will open next Saturday 26th November

I will give you some of the original Spanish text  and you might learn some Spanish from it.

La estación de esquí de Sierra Nevada abrirá el próximo sábado la campaña invernal con 14 kilómetros de pistas (68 hectáreas), 
The ski station in Sierra Nevada will open the winter season next Saturday with 14km of piste (68 hectares)

For more information about the ski station click on this link.

https://granadamap.com/sn/