"Hasta el 40 de mayo no te quites el sayo" = Do not take off your smock until the 40th of May*

We all know there in NO 40th of may, but there you are a good and wise reminder not to jump on summer clothes before it is actually summer!!! Now, how to translate this into English???
There is no exact equivalent to this expression. A similar one that might come in handy could be One robin doesn't make a spring. But, it is a could-do replacement in conveying the same idea, not the real synonym expression.
Fortunately, Isabel Baila has shared the expression used in the North West with us: "Ne'er cast a clout till May be out". A big thank you for that useful contribution ;)