Jestercap occurs at the end of the month of Lamashan, traditionally on the 27th (although a few regions have taken to moving the exact day around slightly so it always falls on the last Starday of the month, allowing people who wish to celebrate in excess to have the following day of rest to recover).
While Jestercap has been embraced with excited open arms by the gnome communities of the Inner Sea region, its original genesis is said to have been in one of Taldor's coastal cities not long after King Aspex the Even-Tongued broke from the nation, significantly weakening Taldor's power and beginning that nation's long decline. The holiday was originally intended to distract the distraught Taldan populace with a night of revelry and comedic entertainment, but the antics of jesters were simply not enough.
Over the course of the first few years, Jestercap evolved from a holiday of observation to a holiday of participation. Today, the holiday is a time where anyone can pull pranks or jokes or japes on companions, on neighbors, and (most typically) on rivals, with the understanding that provided no lasting harm is done, any humiliations inflicted before midnight are to be taken in stride. Of course, come morning the day after, there are inevitably jokes that went too far, and grudges and feuds borne from Jestercap antics have a way of lingering for months to follow.
Followers of Chaldira Zuzaristan, a halfling deity of mischief, treat Jestercap as a holy day and their pranks—often elaborate and extravagant in nature and plotted for months in advance—as displays of their faith.