On New Year´s Eve it is common to either spend your time with friends and family at home or go to a restaurant and have an extensive dinner (Menu de año nuevo). As the latter is very popular you will require an advanced booking.
At midnight it is the tradition to eat twelve grapes – one per bell strike. This will lead to a prosperous year according to the tradition.
PS: Actually, in the late 19th century winemakers had a rich harvest of very good grapes. To increase their revenue they invented this tradition.