Abbreviations for days of the week Is there a set of standard abbreviations for the days of the week for use in astronomy? I couldn't find any reference to it in the The IAU Style Manual (1989).
Wikipedia uses: mon, tue, wed, thu, sat, sun.

ISO 8601 defines an exchange standard for dates based on weeks and days, which looks like this: 2017-W01-3 or this 2017W011 (Year, ISO week number, number of day of week starting with Monday). Not so surprisingly it does not standardize names of days as far as I can tell. 

So you could claim to use ISO-8601 conventions and simply use 1-7 (starting with Monday).

A comprehensive listing of abbreviations of day names for European countries is given by the European Medicines Agency for blistered drugs for the curious.