In reply to Kat93:
Hi Kat.
You should take water, and maybe a snack as well.
Gloves yes, and clothing should be layered as temperatures change fast, and the effort of walking changes clothing necessities as well.
A fully charged mobile phone would work, but you will need to download the map tile first. This can be done through OS maps among others. Because there is no internet in the hills the GPS will work but the blue dot will be on a blank screen - not helpful. I would recommend a map as a back up though, and the compass to go with it. You can pick up maps in any of the outdoor shops there, and some other shops round the country-its a popular area. You can order from OS or the Harveys maps are good for walkers - less confusing detail in complex terrain.
Not actually done that route myself don't know what the nav is like if you are in cloud with low vis sorry. As for snow up at the top - I would be stunned if it's snowy next week.
Weather forecast wise I use MWIS, but this generally seems to be the worst case scenario. Also Met Office. Good luck with no rain in Llanberis! Am there next weekend as well so hopefully your wish will be granted!
Hope this helps, and enjoy your walk.