Most of the Hallmark May Basket sets came in envelopes that could be sent to someone so that they could make and give away the baskets. For some reason it was cheaper to mail them in an unsealed envelope (3 cents) rather than a sealed one (9 cents). Here's a picture of the address side of the envelope.
This is a news release for the first Hallmark May Day baskets. The envelope is postmarked 1953 thought the copyright on the booklet is 1952. It's quite interesting - it's a "newspaper mat" made of pressed cardboard, with the picture and letters embossed into it. As far as I can figure it out, the newspapers that wanted to use it would use the mat as a mold to make their printing block to print the article and picture.
A letter to newspaper editors telling them about the Hallmark May Baskets, and mentioning that 10,000 copies of the May Day Celebrations for Children booklet have been given to schools. The envelope is postmarked 1953. The return address on the envelope adds "Room 3810" to the return address shown on the letter. The public relations firm was Carl Byoir & Associates, Inc.
I recently bought a little lot of Hallmark May Basket items. It included this booklet, May Day Celebrations for Children, copyright 1952. The baskets on the cover of the booklet are those from what I called Set 1, and two copies of that set were included with the kit, so I'm confident that Set 1 is actually the first set, and that it came out in 1952.