The booking cliff: 19 December is the cut-off
For Alhambra visits between December 1 and 19, booking one to two weeks ahead is fine — often less. On December 20 the dynamic changes sharply: school holidays begin across Spain, the ski resort fills to capacity, and Alhambra tickets disappear fast. Book your December 20–31 visit four to six weeks in advance. If you can adjust your arrival by a day or two, December 16–19 is the sweet spot: still quiet, still easy to book, but close enough to Christmas that the lights and market are fully running.