Dan Cooksey wrote:May is a toss up. If you wanted to be sure shoot for July through August. The coastal NW weather can't be predicted, however July and August are usually beautiful. The best price for camping is Stawaumish Chief Campground. Gets crowded on weekends and I believe there are limits to how many days you can stay. There are plenty of private campgrounds that are more pricey but have nice showers and amenities. If you drive north to Cheekamus Canyon (this is the best sport climbing area in Squamish) there is actually some dispersed free camping. Its about 15 miles north of Squamish. Anyway all the climbing is smashed into that central area and you could spend a couple life times trying to get it all done. Just pick up the books both bouldering and routes. Tons of info and a good reference for anything you will need.
+1 to everything he said. May is still pretty dodgy, weather wise, although in some years it is lovely. June is more reliable than May, and July/August are typically quite warm and sunny (in most years). I probably wouldn't plan a road trip there before the beginning of June, and the later in June you go the better. Early July is when the dry summer weather really locks in (in most years...).
Squamish has an enormous quantity and variety of climbing, and the quality is quite high. Also, the camping is nice, the logistics are easy (town is right there), the people are friendly, the scenery is pretty, and there are lots of fun rest day activities. It is a great place to settle in for a while-- the best choice for how long to stay there is "as long as possible". How much time do you have- is this a multi-month odyssey sort of trip, or a 2-week vacation?
If you have 3 weeks or less, plan on spending all of it is Squamish. There is so much climbing that 2-3 weeks is just enough time to scratch the surface. With 4 weeks, I would probably still plan on spending it all in Squamish, or maybe including a stop in Washington or Oregon on the way up. Driving over to Banff adds a lot of extra travel time, and I would only consider it if you are doing a 4+ week trip.
If you end up in Squamish too early in the season and get rained out, you can drive 5 hours to Skaha, which has a large quantity of excellent sport climbing and is in a much drier (semi-desert) climbing on the east side of the mountains. Spring is nice there, while summer is quite hot. I wouldn't drive from CA just for Skaha, but it is a good back-up plan if you gamble on early season in Squamish.
Leavenworth is another dry-side option if you get rained out of Squamish, though the quality of the cragging in Leavenworth is a massive step down from Squamish. I would only go there if the weather forces you to, or if you want to boulder (the bouldering in Leavenworth is good).