we (bad brains mbc) did it last year setting off in the dark to watch the sunrise from the summit.
the llanberis path is fairly easy to follow to the summit – we were blessed with godd weather and fantastic views at dawn.
the weather is the main concern -you need to carry enough gear to be prepared for all conditions. layer up and take a good water and wind proof jacket,carry at least one extra top, more food and drink than you need, at least a foil blanket if not a survial bag just in case , a wooly hat is also a good idea,a whistle, a map, compass and GPS (not essential but handy) in case of low cloud/mist/fog etc.
golden rule is if the weather does turn against you, turn back – don’t be a hero