In English, the days of the week have single-letter abbreviations (M, T, W, etc.) and three-letter abbreviations (Mon., Tue., Wed.). What are the standard ways to abbreviate the days of the week in Spanish?
I don't know if there's any official standard about this, but:
The single-letter abbreviations are:
Note that miércoles is usually written as X, so as not to confuse it with martes.
Regarding múltiple letter abbreviations, the usual way is two-letter abbreviations: