In the somewhat unusual instance where a single digit day abbreviation is needed, the below codes can be used.
|T||Tuesday||Possible confusion with Thursday. Cannot use "S" to distinguish it from Thursday, because of confusion with Saturday or Sunday.|
|R||Thursday||Uses "R" to prevent confusion with Tuesday.|
|S||Saturday||The "S" is the best choice since "T" or "R" can be confused with other days.|
|U||Sunday||The letter "N" is a consonant in Monday, so the "U" is just the least-worst choice.|
Three character day abbreviations (Mon-Sun) and two character abbreviations (Mo-Su) are well-known and unambiguous. A single-digit abbreviation is problematic because of Tuesday vs. Thursday and Saturday vs. Sunday, requiring the least-ambiguous letters, R and U for ThuRsday and SUnday respectively.
The application of these codes is fairly uncommon. The first time we saw them was in the late 80's on dot matrix printouts for M.I.T class schedules. Given the bandwidth, display and storage capacity of modern systems, the use of single-digit day abbreviations is no longer really needed. Lastly, displaying the codes in sequence starting on Monday, i.e. MTWRFSU, does give the reader a sense of what they are looking at without a decode key.