There are some cases where you have to use lowercase letters in Spanish where you would use uppercase letters in English:

1. The days of the week are written with lowercase letters in Spanish: 
• Thank God it's Friday. = Gracias a Dios es viernes. 
• She has an exam on Tuesday. = Ella tiene un examen el martes. 
2. The months of the year are written with lowercase letters in Spanish: 
• Today is May 15. = Hoy es el quince de mayo. 
3. Nouns and adjectives of nationality (but not the name of the country or city) are written with lowercase letters in Spanish: 
• Juan is Peruvian but he doesn't live in Peru. Juan es peruano pero no vive en el Perú. 
4. Languages are written with lowercase letters in Spanish: 
•  I want to learn Spanish. = Quiero aprender español. 
5. The titles of books, literary works, articles, etc., except for the first word are written with lowercase letters in Spanish: 
• I like the book A Hundred Years of Solitude by Garcia Marquez. Me gusta el libro Cien años de soledad de García Márquez.