Mos. on end

YRS

Think of how you might abbreviate the word for annual celebrations on a timeline.

"Mos." is short for "months," and "on end" hints at taking the last letters of that word. The last letters of "months" are "N-T-H-S," but since it's plural, you drop the "S" and take the last letter of the singular form—"month"—which is "H." Wait, no, actually it's a simpler trick: "months" abbreviated is "mos," and the "end" of that is just the last letter, "S." But the answer is "YRS," which stands for "years," because the clue is playing on how both "months" and "years" are time units often abbreviated in crosswords. The real wordplay is that "on end" suggests the final letters, but here it’s just swapping one time abbreviation for another—"mos" (months) clues "yrs" (years) as a misdirection.