It's an Indian summer! (literally St Martin's summer)
'Martino' — mar-TI-no. Stress on the second syllable. 'San' — short, unstressed before the name.
Use in early November (around November 11th, St Martin's Day) when unseasonably warm and sunny weather returns after the first cold. Very culturally specific and appreciated.
'Estate di San Martino' (St Martin's summer) is the Italian equivalent of 'Indian summer' — a warm spell in early November. It falls around November 11th, the feast of St Martin, and is considered a gift of good weather before winter. The phrase carries warmth and nostalgia.
San Martino, estate breve.
St Martin, brief summer.
Poetic abbreviated version — captures the transience
Ancora qualche giorno di caldo, poi arriva il freddo.
A few more warm days, then the cold comes.
Realistic version — acknowledges the brief window before winter
Approfittiamo di questi ultimi giorni belli.
Let's make the most of these last beautiful days.
Bittersweet — enjoy it while it lasts before winter
St Martin's Day (November 11th) is also associated with new wine tasting in Italy — 'a San Martino ogni mosto diventa vino' (by St Martin's day every must becomes wine). The warm weather and new wine create a last celebration before winter.