Days and months stay lowercase in Spanish
In English, days of the week and months of the year always start with a capital letter: Monday, Friday, January, August. Spanish does not do this. Words like lunes, viernes, enero and agosto are ordinary nouns, so they are written in lowercase, just like mesa or casa.
This is one of the most common spelling slips for English speakers, because the capital letter feels automatic. The good news is that the rule is very simple, with almost no exceptions to learn.
In Spanish, the names of days and months are common nouns, not names. They only get a capital letter when a normal spelling rule requires it, which in everyday writing means one thing: the word is the first word of a sentence. In the middle of a sentence, in a date, or after a preposition such as en or de, they stay lowercase.
A useful rule of thumb: if you would not capitalize the word mesa in the same position, do not capitalize a day or a month either.
All seven days follow the rule: lunes, martes, miércoles, jueves, viernes, sábado and domingo. They usually appear with the article el or los, which makes it even clearer that they behave like ordinary nouns.
The twelve months work the same way: enero, febrero, marzo, abril, mayo, junio, julio, agosto, septiembre, octubre, noviembre and diciembre. Unlike days, months normally have no article, and you often see them after en or in a date with de.
You may also see the spelling setiembre. It is correct too, and it is also lowercase.
The typical mistake is simply writing Lunes or Enero in the middle of a sentence out of English habit. The only everyday situation where a capital is right is the start of a sentence, where every word gets one.
Holidays and special dates are different. They are real names, so they take capitals, while the month inside the date stays lowercase. The same goes for names like Semana Santa or Viernes Santo, where the day is part of a holiday name.
What about the seasons? Primavera, verano, otoño and invierno are lowercase too, but that is no surprise, because English writes spring, summer, fall and winter in lowercase as well. You can treat them exactly like you do in English.
In Spanish, days of the week and months are ordinary nouns, so write lunes, viernes, enero and diciembre in lowercase everywhere, including in dates. Use a capital only at the start of a sentence or when the word is part of a real name, such as a holiday like Viernes Santo.