Definition of St Martin's summer in English:

St Martin's summer

noun

British
dated
  • A period of fine weather around 11 November (the feast day of St Martin of Tours).

    • ‘Of course, it might not feel like Winter if one is experiencing a "St. Martin's Summer" -- the equivalent of an "Indian Summer."’
    • ‘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.’