You may not immerse on Sunday night because your
seven clean days will not yet have been completed.
Tevilah on Sunday night would not be valid.
If you cannot get safely to and from the mikveh on Monday night, or if the mikveh is closed, you will need to
delay tevilah until Tuesday night (assuming you can get to mikveh safely that night).
Please feel free to get back to us with any further questions.
B’hatzlacha!