As the name suggests, the festival Pangguni Uttharam is to be observed on the day when the constellation Uttharam is in its ascendant in the Tamil month of Pangguni (mid-March to mid-April). The asterism Uttharam is said to be in its ascendant because, in the Tamil month of Pangguni, the full moon occurs in the zodiac of ‘kanni’, especially in the asterism of Uttharam.
Although, the asterism is acknowledged of its prominence in the month of Pangguni because of its co-occurrence with the full moon, the day for Pangguni Uttharam does not always occur with the day for full moon day fast. Generally, the day of full moon in the month of Pangguni falls one day before, on the same day or one day after the day when the asterism of Uttharam occurs. Therefore, as the name of the festival indicates the asterism of Uttharam should be effectively prevalent, when the festival is celebrated.
The Almanac prepared by Sri Ragunatha Iyer of SriLanka, indicates that the time of occurrence of the asterism Uttharam in the month of Pangguni is from 9.39 am on Friday 30.03.2018 to 9.04 am on Saturday 31.03.2018. It is very clear that the asterism Uttharam is effectively prevalent on Friday 30.03.2018; the asterism is only until 9.04 am on Saturday. Therefore, it is clearly evident that it is proper to observe Pangguni Uttharam, on the day, the whole of which is, characterized by the presence of the asterism, Uttharam.