Dating patterns is an ongoing issue (well, for those of us who care). McCall['s] has been good about dating since the 1920's, and Simplicity did for a little while in the 1940's, but most patterns before the 1980's didn't have dates on the envelopes. Most postal stamps on bulk rate didn't have dates, either, and catalogs weren't meant to last forever and are kind of hard to get.
I got this on eBay.
This also allowed me to date it: There is a mention of rolling bandages for the Red Cross, although that could take place even in peacetime. Several events--sewing circle, PTA meeting--are expected to take place on Thursday, March 21, which narrowed down the year since March 21 won't be on a Thursday every time.
Finally, there is a partial obituary for an "Oscar W. Peter---", who was born on [month] 8, 1895. A quick Ancestry.com check turned up an Oscar W. Petersen (December 8, 1895 in Denmark - March 19, 1918 in Chicago, Illinois), and the street address matches.