1. As a noun:
* "Two days' notice" is the grammatically correct form. This is because "notice" is a noun and the possessive "s" is used to show that the notice belongs to two days.
2. As a verb:
* "Give two days' notice" is the grammatically correct form. This is because "give" is a verb and "two days' notice" is the object of the verb.
3. In a sentence:
* You should give two days' notice before you leave.
* I received two days' notice of the meeting.
In summary, the correct grammar for "two days notice" is "two days' notice" when used as a noun and "give two days' notice" when used as a verb.