Is it permissible to postpone the completion of the 'Umrah rituals until the following day after arriving in Makkah, or is it obligatory to assume the state of Ihram from the Miqat on the plane and make the intention for 'Umrah?

It is permissible to postpone the completion of the 'Umrah rituals after entering into a state of ihram from the miqat, while avoiding the prohibitions of ihram. It is not permissible to pass the miqat without entering into ihram for one who intends to perform Hajj or 'Umrah. So, whoever passes it without ihram must return to enter into ihram; otherwise, a sacrifice (dam) is due from him.