From AMS Glossary
(Also called Allhallow summer, All Saints' summer.) In English folklore, an old name for a period, like Indian summer, of unseasonable warmth, supposed to occur on the eve of All Hallows day (All Saints Day, November 1). It is mentioned by Shakespeare, but its use appears to have died out. More frequently heard today are references to such as St. Luke's summer, St. Martin's summer, Old Wives' summer.