The Zenith of Zarthus starts at noon on the 20th of Zarthus' Zodiac month and extends to noon on the 21st coinciding with the 20th and 21st of the Month of Hayseed on the secular calendar.
"This late in the month of Hayseed is a good time for an early harvest festival for this crop or that crop, so a lot of princes set up national holidays around this time. If the local ruler is nominally Lantern-friendly or wants to publicly appear to be Lantern-friendly there might be a feast day on Zarthus' Zenith rather than merely near it. Either way, nothing is stop us from having a feast with our parishioners assuming we have enough food on hand. If the local Lanterns lean towards the Patrons then there is almost certainly going to an art exhibition. Sculptors, painters, and potters like to show off their latest work and musicians and acting troupes commonly stage performances at this time, either to please wealthy patrons they already have or to try to solicit new supporters. -Zajac, Fumayan Lantern