the next day on the next day

[ripley]

Hi, I'd like to know if it is necessary to use the preposition on in these sentences.

1on the next day we left
2the next day we left

3on the following day we left
4the following day we left

5on Monday we will leave
6Monday we will leave

Thanks Rip.

[Tdol]

No- you can get by without the preposition, though some might not like #6. I wouldn't write #6.