I will be going again in April this year.

As RobbieRunner advises I also always fly in to Seattle find it much cheaper than YVR from the north of England.

Combined the trip once with a trip via the Olympic peninsula and Port Angeles, via Victoria, Nanaimo to Whistler and back to SEA. This was the 2nd week in April. Went from sunbathing on a beach in Victoria to Skiing in Whistler in a couple of days. Good times!

I will be there again in the first week of April this year but I have been there as late as mid-April in the past.

I found that the temperatures were on the high side and the lower runs were quite slushy by the end of the day. The top of the mountain (above Olympic) remained good for most, if not all of the day.

Worst case scenario is when it is snowing at the top of the mountain and raining at the bottom. I experienced this once and it was not pleasant.

On the plus side the snow seems to pretty amazing again this year so there should be a massive base built up by April.