Days of the week: using 'el' and 'los'
English speakers have a strong instinct to translate word for word. When talking about days of the week, this leads to a very common beginner mistake. Because we say on Monday in English, it is tempting to say en lunes in Spanish.
However, Spanish never uses a preposition to say that an event happens on a specific day. Instead, Spanish uses the definite articles el and los. You will also notice that days of the week, like lunes and martes, are always masculine and are not capitalized.
When an event happens on one specific day, you simply put el in front of the day. If you want to say I will see you on Friday, you just say I will see you the Friday.
If something happens regularly, like on Saturdays or every Wednesday, you use the plural article los.
Pay attention to the spelling. The weekdays from Monday to Friday already end in an s, so the word itself does not change in the plural. You just change el to los.
For the weekend days, which end in vowels, you need to add an s to make them plural.
You only use el and los as a replacement for on. If you are simply stating what day it is today, you do not use an article at all.
You can also use other prepositions with days of the week to show starting and ending points, like hasta (until). When grouping days together with de (from) and a (to), the article is usually dropped.
The rule of thumb is simple. Never translate on as en before a day of the week. Use el for a single occurrence and los for a recurring habit, and remember to keep those days lowercase.