And Mars may be the kicker for the overall length. With a 780-day synodic period, 21 periods match exactly to 16,380, or 20 cycles of 819. Venus needs seven periods to match five 819-day counts ...

it matches up with all synodic periods of the visible planets, explaining it as a planetary astronomical cycle. What is interesting here is that the Mayan counting system is base-20 (vigesimal).