Hmm, May in Alberta can be a very interesting time.
Most of the time it is nice weather, anywhere in the range 15-25, however we do still get snow in May sometimes, and some rain.
However I went to Banff last May with my family and we had a great time, no snow, no rain, nice and sunny, if a bit breezy. It is a beautiful place to visit at anytime of the year.
If you have the opportunity to go I say go for it, just bring sweatershirts, jeans, capri's and t-shirt to cover all the bases, and don't forget your bathing suit for the natural hot springs.
HTH
Vickie