The functions are only checking the day and are not converting into the german time zone first.
"2025-05-28T23:00:00.00Z" is already 2025-05-29 in Germany and with this request Holiday should be true as it is Christi Himmelfahrt.
const date11 = new Date("2025-05-28T23:00:00.00Z");
console.log( Feiertage.isHoliday(date11,'ALL') );
