Aha! I thought, as I read this entry in Emma Kennedy’s blog. She mentions the expression "Ne’er cast a clout til May is out", and I thought I could finally establish a logical pattern of equivalence between French and English expressions by linking it to the expressions April showers and Giboulées de mars discussed a few weeks ago. Indeed, the equivalent French expression is "En avril, ne te découvre pas d’un fil!" (Never cast a thread until April is dead). English April becomes March in French, whereas May turns into April. The English are a month behind, that’s all! And that’s probably due to the fact that being further north, spring weather takes longer to arrive on these shores.
Then I found the Nature Detectives website which insists that "the saying, ‘Ne’er cast a clout till May is out’ is thought to refer to the hawthorn blossom, not the month and was good advice that summer hadn’t really arrived until the blossom was in flower." No sooner had my theory been formulated that it was thrown into question! I had another look around, hoping this would be a rogue site written by people with not a care for linguistic logic, but I found that Practical conservation online and several other sites agreed. But then, if May really is a blossom, shouldn’t the saying sound more like "Ne’er cast a clout til mays are out"?