Antonyms of adj red-letter

1 sense of red-letter

Sense 1
red-letter -- (memorable for being a special occasion; "on red-letter nights a large gathering greeted the celebrity")

INDIRECT (VIA unforgettable) -> forgettable -- (easily forgotten)

Similarity of adj red-letter

1 sense of red-letter

Sense 1
red-letter -- (memorable for being a special occasion; "on red-letter nights a large gathering greeted the celebrity")
       => unforgettable (vs. forgettable) -- (impossible to forget)