Which word should always begin with a capital letter: sun or sunday?
You use should when you are saying what would be the right thing to do or the right state for something to be in. 应该
If you always do something, you do it whenever a particular situation occurs. If you always did something, you did it whenever a particular situation occurred. 总是; 每次
The Usage of capital letters
Use capital letters for the names of people, places, planets, days of the week, titles of rank or relationship (when joined to person’s name, e.g., Sergeant Smith, Uncle Fred), months, holidays, departments, clubs, companies, institutions, bridges, buildings, monuments, parks, ships, hotels, streets, historical events, and documents. These are known as proper nouns.
Do not use a capital letter for a common noun, unless it starts a sentence. A common noun is the word we use for something, e.g., dog, ship, auntie.