Days of the week can be either uncountable or countable depending on how they are used. When a day of the week is being referred to generally, it is used as an uncountable noun, i.e. no indefinite article is used with them.
Sunday comes before Monday.
Sunday is a day of rest.
Monday is the most unpopular day of the week.
Many people don’t like Monday because they have to go back to work.
When a day of the week refers to a specific thing or situation, it is used as a countable noun, i.e. either the indefinite article is used or the noun is pluralized.
Jim thought that that is was going to be a normal Sunday, boy was he wrong!
The bar serves five-dollar pints on Mondays.
The bank is open for business on Sundays.
The article promised 3 perfect ways to spend a Sunday in London.
To learn more about nouns that can be countable or uncountable, please read the 2nd Exception to Rule 3.1)