Unfortunately, until the
seven clean days have been completed,
tevilah is not valid. Therefore, an immersion Thursday night would not be valid, and you would remain in
niddah. Not telling your husband would not help.
You can try to contact the mikvaot in the locations where you will be Motzai Shabbat and/or Sunday night to see if you can schedule an appointment at a time that will work for you.
If neither of those options work out, unfortunately you will need to
delay immersion until Monday night.
Please feel free to get back to us with any further questions.
B'Hatzlacha!
