A period of fine weather around 11 November (the feast day of St Martin of Tours).
〈英，旧〉十一月小阳春（11月11日 图尔的圣马丁节左右风和日暖的一段时期）
Example sentencesExamples
Poins calls Falstaff ‘Martlemas', perhaps alluding to St Martin's summer (compare Prince Harry's ‘All-hallown summer’); or to Martinmas beef, fattened for slaughter by that date.
Of course, it might not feel like Winter if one is experiencing a "St. Martin's Summer" -- the equivalent of an "Indian Summer."