Which grammatical case indicate the ending "-na"?
Examples:
1. Mitä sinä aiot tehdä viikonloppuna? (What are your plans for the weekend? )
2. Mitä sinä aiot tehdä huomenna? (What are you going to do tomorrow?)
3. Mitä sinä aiot tehdä maanantaina? (What are you going to do on Monday?")

Exception
Mitä sinä aiot tehdä lomalla? (What do you do on vacation?)
Questions:
1. Which grammatical case indicate the ending "-na"?
2 .Can you think at other exceptions?

Question

Which grammatical case indicate the ending "-na"?
Examples:
1. Mitä sinä aiot tehdä viikonloppuna? (What are your plans for the weekend? )
2. Mitä sinä aiot tehdä huomenna? (What are you going to do tomorrow?) 
3. Mitä sinä aiot tehdä maanantaina? (What are you going to do on Monday?")

Exception
Mitä sinä aiot tehdä lomalla? (What do you do on vacation?)
Questions:
1. Which grammatical case indicate the ending "-na"?
2 .Can you think at other exceptions?

Answer

The -na/-nä ending is the ending of the essive case. It has many equivalents in English:
1) at
     kotona (at home)
2) in
     vuonna 2017 (in 2017)
3) on
     maanantaina (on Monday)
4) as
     Työskentelen opettajana. (I work as a teacher.)
5) no equivalent in English
     Hän on raskaana. (She is pregnant.)
     ensi kesänä (next summer)

You can read more about the essive case on this site:
https://en.wikipedia.org/wiki/Essive_case