If a festival occurs during tharpanam days, we should prepare festive food and offer it to Perumal.
Notes:
It is a customary practice that Sankramanam is celebrated like a festival. At beginning of a new month, it is carried out as usual by performing tharpanam. So, this question does not bear anything with regards to Sankramanam.
When a festival occurs during the new moon day (Amavasai), the festival must be celebrated. One might have this specific query when Deepavali and Amavasai occur together.
Sometimes birthdays or other festivals can occur on the day of a tharpanam. In such instances, tharpanam should be done first followed by tiruaradhanam and offering of special festival food to Perumal.