What does the cryptic crossword clue "Marking joyous day, landlord on the left? (3-6)" mean?

The definition is "Marking joyous day, landlord on the left?". The wordplay uses a double definition. Double definition The answer is REDLETTER.

Marking joyous day, landlord on the left? (3-6)

Guardian Cryptic #29460 | 8 Down | 2024-08-13 flower