Meaning: Until 40th of May dont take off you smock.
When to use: Sometimes by the end os May is already too hot in Spain, so you can get the feeling that summer has arrived. However most likely some cold & rainy days will come. We use this sentence to warn & remind our friends that few cold days may still will come. Summer usually settles in Spain from June 9.
Where is coming from: We don´t know exactly when it became popular, however there are people in Madrid who think that the saying has nothing to do with the weather, and refers to a successful brothel called “40 de Mayo” in Madrid. According to this theory, the intention was to indicate to those from the rest of Spain not to take their clothes off until they reached this brothel, since it was the highest quality one.
Funny Reply: It is usually answered to the saying, as a joke, with the phrase “Y si eres de Albacete, hasta el 47” (“And if you are from Albacete, until 47 (May)”). This refers to the low temperatures that are reached in this city of La Mancha.