In reply to mason_7:
it can snow on the summit plateau any time of year, and patches of snow survive in the north face gullies year round. typically by the start of august the whole tourist route and cmd arête route will be snow free, but last year a very large snow patch was still covering the tourist route just below the summit in late august. at the moment it is still full winter above 1000m in the highlands and the snow depth at the summit cairn is around 4m, so it will survive for quite some time yet!
climbing wise many of the rock routes can be done on a warm sunny day from spring onwards, but accessing the routes will require snow approaches. the castle area tends to be completely snow free first
Post edited at 14:11