When is generally a good time to start?

We crossed from Washington into BC on May 1st. of this year. The camping guide we were using, in addition to the Milepost, listed many of the camp grounds opening either May 1st or May 15th. But several we had planned on staying at were still closed when we got there. This resulted in putting us way ahead of schedule as many days we had to drive further to find a suitable campground. Many times we had the whole campground to ourselves.

We woke up to dusting of snow one morning, but for the most part the further north we got, the warmer it got. By the time we arrived in Fairbanks on May 19th,it was in the 80's.