Hello and welcome to HCM.
Thank you for your query.
Primolut N will successfully help in postponing your period for as long as you wish to. The instructions are as follows:
-Take the medication as advised by the doctor, till you wish to postpone your periods, for as many days required
-Once you are done with the reason for postponement, you can discontinue the medication, and you should get your period within 3-7 days after discontinuing the medication. In rare cases, women get their period 10-15 days after discontinuing the medication
Now coming to your next query, you have missed a dose; do not worry, and just continue to take the medication, you should not get your period most probably.
If you wish not to have your period till the 21st, then continue taking the medication till the 21st, and discontinue from the 22nd. You should get your period within 3-7 days after discontinuing the medication.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications at: http://doctor.healthcaremagic.com/doctors/dr-shoaib-khan/64581 . I would gladly help you.
Best wishes.