In the puṣkara days the tradition holds that Guru's power flows into that river like a current — bathing, japa, dāna and sādhana done on its banks are held to bear multiplied fruit. And for those born under that rāśi, or with Guru placed there in the birth-chart, the season is counted kinder still.
Every first entry of Guru into a new sign opens that river's puṣkaram — the festival proper is the twelve days from the entry. Dates are found from the true Jupiter (Indian civil dates); where Guru retrogrades back over a boundary, the festival is kept at the first entry.
The West knows the same clock as the Jupiter cycle — twelve years to round the zodiac, the classic rhythm of growth and opportunity, and the "Jupiter return" at ~12, 24, 36… Here that same walk is made a geography: each year of Jupiter's round belongs to a river.
The puṣkara-cakra is old doctrine — the classical compendia treat the wheel of Guru's transits and its river-festivals among the fruits of the planets' wanderings. It was built to knit the land together: every region's life-river receives its festival in turn, once in twelve years.
The festival rides Guru's entry into a sidereal sign — so, like the Nine Lords of the Year, this is a nirayana reading: the ayanāṃśa chooses the very date of the puṣkaram. The twelve-year rhythm is frame-free (Jupiter's period cares nothing for zodiacs); only the moment each river's turn begins is frame-bound. Arka reads it from the true sidereal sky, as the almanacs do.