If you did a hefsek taharah on Monday before sunset, and then did a moch dachuk, then you go to the mikveh the following Monday night after nightfall.
In fact, as long as you did your hefsek before sunset on Monday, then even if you didn't do a moch dachuk on time (or didn't do it at all), you would still go to the mikveh the following Monday. The moch dachuk is an important custom, but a hefsek taharah is generally valid without it.
If you did not do a hefsek before sunset on Monday, then you cannot go to the mikveh until Tuesday night.