In current Spanish when we have to abbreviate the days of the week using only one character, in most places (but not everywhere) to tell apart martes (Tuesday) from miércoles (Wednesday) we use 'M' and 'X', respectively.

I have read several stories about the origin of the 'X' abbreviation. One of them states that as miércoles originates from [dies] Mercurii, and as this word comes from merx, we can tell apart Mars (that originated martes) from merx using the last 'X'.

But if this story is true, this abbreviation could have been used already in the times of the Roman Empire. So my question is: how did the Romans abbreviate the names of the days of the week? Did them by any chance use the X for dies Mercurii to tell it apart from dies Martis?

  • Good question! Perhaps before answering this one has to describe how (if at all!) the days of the week were used in antiquity. My impression is that they are a later addition to the calendar system. It might make sense to ask a prequel question.
    – Joonas Ilmavirta
    Jul 1, 2019 at 9:13
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    @JoonasIlmavirta better done that said. :-)
    – Charlie
    Jul 1, 2019 at 9:43
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    @JoonasIlmavirta: the seven-day planetary week is first alluded to by Tibullus (1st century BC).
    – fdb
    Jul 1, 2019 at 11:01
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    @fdb Would it be possible to write up an answer about the relevant passage from Tibullus to the other question about the days of the week?
    – Joonas Ilmavirta
    Jul 1, 2019 at 12:52
  • Fun fact: in Chile it is not uncommon to abbreviate miércoles as W (for Wednesday): DLMWJVSD
    – Rafael
    Jul 2, 2019 at 13:04


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