¡La Esquina de la Gramática! Rule 9

Months, days of the week, seasons of the year… lowercase or capital letter?

In English, the first letter of the days of the week, months, and seasons is a capital letter, but that is not the case in Spanish. This is a common mistake I see even among Spanish-speaking people living in the United States for an extended period of time, as sometimes they tend to mix the grammar rules for both languages.

Examples:

Anteayer fue martes, 2 de mayo.

Esta primavera ha estado lloviendo mucho.

The only exception is when they are part of names that require to be written with capital letters as holidays, roads, buildings, etc. For example:

Viernes Santo

Plaza del Dos de Mayo

Hospital Doce de Octubre.

Source: http://www.rae.es