The days of the week are probably one of the first things you learn, not just in Spanish but in any language. The days of the week are not capitalized in Spanish – unless they appear at the beginning of a sentence. In Spanish, the days are all masculine; 'el lunes' (Monday). To make a day of the week plural, add an s (if it doesn't already end in one), for example, 'los domingos.' Sábado and domingo are the weekend days (Saturday and Sunday, respectively).

Source: Mondly