Our family used to follow this mantra too maydup. From the first of June onwards we were allowed to leave our vests off. The bedding plants were planted and summer was upon us. Still got vests at the ready this year though.
Apparently the old verse refers to the Blossom of the May tree (Hawthorne) rather than the month of may Our May tree blossom is still not out!!!!! (but I have cast a few clouts today) ;o)
Silly me, I casted my clouts a couple of weeks ago cos it was quite warm, and then it got cold, but quilt had been put away so had to pile covers on the bed, still got them, not that warm yet here in Ibiza.