intercalation
insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases
en-US/ɪnˌtɝkəˈleɪʃən/
en-GB/ɪntɜːkəleɪʃən/
insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases