The following are Spanish weekdays from Monday to Sunday:
Monday = lunes
Tuesday = martes
Wednesday = miércoles
Thursday = jueves
Friday = viernes
Saturday = sábado
Sunday = domingo
Origin of the words
The Spanish weekdays look and sound very different from the English ones, so you generally just need to memorize them. One thing that might help, but may not be immediately obvious, is the fact that the days of the work week (Monday to Friday) are named after the Moon and the first four planets (besides the Earth) – unfortunately in no logical order:
Monday = lunes = after la luna (the Moon)
Tuesday = martes = after Marte (Mars)
Wednesday = miércoles = after Mercurio (Mercury)
Thursday = jueves = after Júpiter (Jupiter)
Friday = viernes = after Venus (Venus)
Note: Besides celestial bodies, these names also refer to Ancient Roman gods. In fact, the English weekday names are derived from the same gods, or more precisely, their Germanic equivalents: Monday after the Moon, Tuesday after Tiw (Germanic god of war, like the Roman Mars), Wednesday after Odin or Woden (Germanic equivalent to Mercury), Thursday after Thor (god of lightning like the Roman god Jupiter), and Friday after Frigg (godess of love and beauty like the Roman Venus).
The Spanish weekend days are all about resting and religion:
Saturday = sábado = from the Hebrew word sabat (rest)
Sunday = domingo = from the Latin word Dominus (Lord)
Rules for using Spanish weekdays
Unlike English, the Spanish weekdays are never capitalized (except, of course, at the beginning of a sentence). It is the same for Spanish months.
All the weekdays have masculine gender, which means they have the articles el (definite singular), los (definite plural), un (indefinite singular), or unos/algunos (indefinite plural).
When saying that something happens on a particular day of week, an article should be used before the weekday. Choosing the singular el or the plural los changes the meaning (this Monday vs. every Monday):
No trabajo el lunes. = I don’t work this Monday.
No trabajo los lunes. = I don’t work on Mondays.
Notice that unlike English (Monday vs. Mondays) the word for the weekday itself (lunes) remains the same in singular and plural form. This is the case for all the weekdays from lunes to viernes, as all end with -s. On the contrary, sábado and domingo add an -s in the plural form (sábados, domingos).
From Monday to Friday
The prepositions to be used when something is happening from one weekday until another are:
de … a … = from … to …
Trabajo de lunes a viernes. = I work from Monday(s) to Friday(s).
Notice there are no articles after the prepositions.
You will often find this “de … a …” format on shops’ opening hours in the Spanish speaking world. For example, a store may be open (abierto) “de lunes a sábado” = from Monday to Saturday.
Abbrieviations and notation
Opening hours, calendars, and similar materials often abbreviate weekday names. There is no single unified rule how to do that and you can find two-letter or three-letter abbreviations, as well as sets with variable number of letters for different weekdays. Some of the most common sets of abbreviations are the following:
lun = lunes
mar = martes
mie = miércoles
jue = jueves
vie = viernes
sab = sábado
dom = domingo
Notice the accent marks missing in “mie” and “sab”. This is not a universal rule – sometimes they are present, sometimes not.
A dot is often written after the abbreviation (e.g. “lun.”, “mar.”, “mie.” etc.).
The abbrieviations are sometimes capitalized (e.g. “Lun”, “Mar”, “Mie” etc.).
Sometimes the abbrieviation for miércoles is miér. (four letters), while three-letter abbrieviations are used for all the other days.
Also very common (for example in calendars) are two-letter abbrieviations for all days, also often capitalized:
Lu = lunes
Ma = martes
Mi = miércoles
Ju = jueves
Vi = viernes
Sa = sábado
Do = domingo
Single-letter abbreviations are less common than the above, but also sometimes used. Because martes and miércoles both start with M, the latter uses the letter X (why X?):
L = lunes
M = martes
X = miércoles
J = jueves
V = viernes
S = sábado
D = domingo