From the release:
Every calendar is peppered with holidays and annual causes for celebration. From local village festivals to grand revels that consume empires, no campaign setting is complete without holidays to mark the passage of time with jubilation and ritual observance. In addition to adding greater realism and flavor to campaign, holidays also offer unique benefits to adventurers who celebrate during downtime or take a break from their quests to get into the spirit of the season.
"Festivals & High Holy Days" contains entries for a dozen holidays and details on how to use them in role-playing games, to include the benefits they can bestow upon those participating in them.
This mini-sourcebook has been specifically designed for use with the Pathfinder Roleplaying Game but can be used with any OGL-based game pretty much as-is and easily adapted to any related or successor systems.