Definite and indefinite plans
Native English speakers sometimes make social invitations with a very indefinite future time reference, such as : Let’s go out for coffee sometime. This is meant more as a gesture of friendliness than as an actual invitation. Students should be aware that a real invitation always has a specific time with it: Would you like to come to my house for dinner on Friday? If a speaker makes a vague statement like : You should come to my house sometime, he or she would be very surprised if the listener actually came without further discussion.

Source: Globallink , Teacher’s book

Definite and indefinite Time Expression
Definite
Tomorrow
Last year
The day after tomorrow
Today
In two day
The week after next

indefinite
One of these day
Someday
Sometime
Sooner or later











Source: Globallink M. 6