When does mild change from warm to cool?

Was listening to the weather forecast this evening and the announcer said that the next few days would be mild. This got me thinking, in winter when we say it is mild we mean warm for the time of year. Yet in summer when we say it is mild we mean that it is cool for the time of year.
So when does this change in meaning take place? Is it when the clocks change? Is it buried deep within the human psyche? It appears to morph from one meaning to the other; yet we all know what the forecaster means; how, why? (And why am I bothered?)