Formal:
* Ko te Rāhina tēnei. (This is Monday.)
Informal:
* Rāhina tēnei. (Monday this.)
Breakdown:
* Ko - is (formal)
* te Rāhina - Monday
* tēnei - this
You can also use ināianei (now) for a more specific "today":
* Ko te Rāhina ināianei. (It is Monday now.)
* Rāhina ināianei. (Monday now.)
The choice between formal and informal depends on the context and your relationship with the person you are talking to.