Several points preventing this project to pass as a usable library:

• extensible definitions - in some countries the set of holidays can easily change year-to-year, and causing a rebuild/redeploy due to such external change is unprofessional
• efficiency - current design causes a lot of redundant method invocations. Yes, so far we prefer simplicity over cleverness, but soon we will be forced to count every nanowatt of CPU due to green movement;
• standard API - I would expect to see implementation of standard java.time.temporal interfaces such as TemporalQuery, TemporalAdjuster. Perhaps not widely known, but very useful
• expressive API - often yes/no answer is insufficient, requiring to determine which holiday occurs on a given date. Also, queries like "when is the next holiday", "find next business day after a certain date" are to be expected in almost any application that works with a concept of bank holidays.