Q. What is the stance of CMOS on single-letter abbreviations for days of the week? In US higher education, the single-letter abbreviations (M, T, W, R, F, S, U) are ubiquitous, though I find no mention of these abbreviations being codified (ISO uses numbers). In prose, I often find myself using these abbreviations in lists of upcoming deadlines.
A. CMOS doesn’t cover those single-letter abbreviations, but we’d consider them to be a convenient shorthand. In the same way, “1/5” can be a handy way of writing January 5.
But because the meaning of “U” and “R” especially may not be clear, and because “1/5” means May 1 for many readers, we don’t recommend either of those forms in formal writing. Where brevity is the main consideration, our preference would be for Su, M, Tu, W, Th, F, and Sa. For those options and two others, see CMOS 10.40. For all-numeral dates, see CMOS 9.35.