A Vishwakarma idol being taken to the pandal on Tuesday. Picture by Amit Datta

You have to reach out for the almanac to find out the Durga puja dates, but not so for Vishwakarma puja because it’s one Hindu festival that doesn’t need to follow the tithi (lunar day) system.

Just as Christmas is celebrated on December 25, Vishwakarma puja is most likely to fall on September 17. Even if it varies, it does so by just a day, according to both the Suryasiddhanta and Bisuddhasiddhanta schools of almanacs.

Unlike all other rituals (except Kartik puja) that are based on tithi, the worship of the god of manufacturing is determined by the transit of the sun. “Vishwakarma puja is scheduled on the last day of the Bengali month of Bhadra, more specifically on Bhadra sankranti. That is when the sun transits from Singha (Leo) to Kanya (Virgo) sign,” explains Pulak Bhattacharya of Gupta Press, the 139-year-old almanac that follows Suryasiddhanta. “The sun’s transit into Virgo is what determines the puja date for us, too,” adds Manoj Lahiri, who subscribes to Bisuddhasiddhanta.

There are usually 156 days of the Bengali year in the five months before Vishwakarma puja, calculates Bhattacharya.

In some years, one of the five months may have a day more or less because the number of days in the Bengali month fluctuates between 29 and 32. That is when the last day of Bhadra gets pushed back or forward by a day.

A scan of the last two decades of almanacs at the Gupta Press library revealed just two years when the puja has happened a day later — on September 18. If Asharh and Sravan had 32 days in 2007, Asharh and Bhadra had 32 days in 1995. Usually only one month has 32 days in the first half of the Bengali year.

For Bisuddhasidhanta, the departure during the same period happened seven times, bringing the puja forward to September 16 on each occasion.

In contrast, Durga puja dates vary by a long way. “Since it is tithi-based, we have to take into account the phenomenon of malamasha, a month with two new moons. That renders the entire month inauspicious for puja. The phenomenon occurs every 30 to 33 months,” Bhattacharya says. So while Puja starts on October 6 (Saptami) this year, next year the wait will be over as early as September 25.