This is not a First Day Cover, but rather an Event Cover commemorating the Scout-O-Rama held in Fremont NB on May 8 1965, a little over a month after the Crusade Against Cancer stamp was released.  I really have no idea what a "Scout-O-Rama" is, but it obviously has something to do with the Boy Scouts of America.  Event Covers, such as this one, can feature just about any stamp the organizers of the event choose to use.  I find it interesting that, of all the stamps available at the time, they chose the Crusade Against Cancer stamp for their cover.  If you can provide any other information about this nice Event Cover, please drop me a line at smith@in.net.