I've parked at Pen-y-Pass twice, once in June and once in August, both times coincided with school holidays, Friends, who live on Angelsey, advised that we get there for seven o'clock. On both occassions the car park was at least half full by then.
As Chewie intermates, the grass verge by the Pen-y-Gwryd hotel could be a good alternative, and far cheaper, option. But it does add an extra 100m of ascent to the start of your day.