'days' is a plural form of the noun 'day' and refers to a unit of time, equal to 24 hours.
'Days' is often preceded by a modifier (a word, phrase, or clause that describes another word) to create a phrase that becomes a noun, as in 'a day of work' or 'three days of rain.' 'Days' is also used to refer to periods such as 'Biblical days,' 'Golden days,' 'Dark days,' 'hard days,' etc. 'Days' can also refer to particular days delimiting dates, such as 'Christmas Day,' 'Halloween Day,' or 'Thanksgiving Day.'
1. 'It's been three long days since I've been able to see my family.'
2. 'The days before the party were filled with excitement.'
3. 'The days of classical music are long gone.'
when referring to a particular month/date, do not use 'Days' when referring to the ordinal number alone; instead, use 'day,' as in 'March 28th day.' To indicate periods of time, you will usually have to add a modifier before 'days,' such as 'long,' 'torrid,' 'rainy,' etc.