I’m planning to leave on the fifth (=the fifth day of the month).
Phrases
PHRASES FROM THE ENTRY►fifth wheel
Analytically this substantive meaning is redundant, a fifth wheel on the coach.
1coming after four other things in a series: in the fifth century her fifth birthday2 fifth wheel American English informal someone who is not wanted in a particular group of people: They’d made her feel like a fifth wheel.—fifth pronoun: I’m planning to leave on the fifth (=the fifth day of the month).—fifthly adverb