We had been leaving in late April or early May for the annual trip to Alaska. In 2013 the son left about 25 April and encountered severe freezing weather and snow, had to lay over a day or two. We were about 2 weeks later and encountered snow in most campgrounds and snow on the road from Glenallen to Anchorage and south on 25 May.
2012 was a pleasant trip with no issues.
2014 had snow early May in Alberta and most campgrounds were closed.
We do this annually. A few years have been fine, but more years than not we have driven in snow and had freezing temps both overnight and during the day. Not every day, not all day, but enough. And that is for early, but usually not the first, of May.
Check Great Alaska Holidays, they go forth about May 10 from Iowa. They are likely full for 2015 but can provide info.
I