You must do all the things as per shastra, you can not skip anything just like that without valid reasons.
Valid reasons as per shastra are: 1) Inability of kartha due to ill-health and 2) Theetu
Combining of Masikam is possible if a masikam was skipped due to the above valid reasons
one should not plan for skipping it.

The 27th Oonam is a colloquial name and its actual name is 'Adhya Oonam' should be done after 27 days. So, it is not possible done on 27th day.

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