Is it better to postpone a marriage scheduled for Thursday, knowing that a relative of mine passed away on Monday? And is it true that holding celebrations three days after a condolence period is permissible in Islamic law?

There is no prohibition against holding weddings for a specific period after the death of a deceased person other than the husband. The default rule in this regard is permissibility and lawfulness. However, it is recommended to consider the feelings of the deceased's family by postponing the celebration for three days for condolences. As for after the three days, there should be no delay, as showing grief is not permissible after this period for the deceased's family or others. Therefore, there is no objection to holding the wedding celebration at the mentioned time.