Learn Finnish: Minulla ei ole autoa lauantaina. - I do not have a car on Saturday.

The -na ending is used to express 'on a certain day' (this is called the essive case). So, lauantai means 'Saturday', but lauantaina means 'on Saturday'.

Will the ending for days of the week always be -na, or does it change to -nä?

Because of vowel harmony, this grammatical ending is normally either -na or -nä. However, since every day of the week in Finnish ends in -tai (which contains the back vowel a), they all take the back vowel ending. So for weekdays, it will always be -na!

Why is the word for car autoa instead of just auto?

Because this is a negative sentence. When you say you do not have something (Minulla ei ole...), the thing you do not have must take the partitive case. We add an -a to auto to make it autoa.

I remember ei ole means 'is not'. How does this mean 'I do not have'?

Finnish does not have a verb for 'to have'. To say you have something, you use the ending -lla (meaning 'on') on the person, plus the verb 'to be'. Minulla means 'on me'. So Minulla ei ole literally translates to 'On me is not'.