This is the `holidays` Python library, a fast, efficient tool for generating country-, province-, state-, and financial-market-specific holiday calendars on the fly. It supports over 250 ISO 3166-1 countries (with subdivisions) and 25+ financial exchanges (NYSE, NASDAQ, LSE, etc.), using a mixin-based composition pattern to combine calendar systems (Gregorian, Islamic, Hebrew, Chinese, Hindu, Buddhist, etc.) with localization in dozens of languages. The library is designed for integration into applications that need to check whether a date is a holiday, retrieve holiday names, or export calendars to iCal format. It is not a data research project; it is a well-engineered Python library with a clean public API (`holidays.US()`, `holidays.country_holidays('US')`), extensive test coverage (100% code coverage enforced), and a robust observed-holiday rule system. The codebase demonstrates high craftsmanship through its reusable mixin pattern, configurable observed rules, and a generic iCal exporter. It is actively maintained with frequent commits, CI/CD pipelines, and dependabot integration.