It can be used as both countable and uncountable nouns. The basic meaning of day is "a day", which refers to a period of 24 hours. It is a countable noun, and it can also mean "daytime, daytime". Day 18 can be a countable noun or an uncountable noun, generally without articles. Day can also be used as the solution of "period, era and life span", and it is often used in the plural form.