Service members on active duty are paid on the 1st and 15th of each month. When either falls on a weekend or bank holiday, pay goes out the business day before, so some months' pay arrives in the previous month.
The payday scheduled for Friday, Jan 1, 2027 (New Year's Day) is deposited on Thursday, Dec 31, 2026, so it counts toward 2026.
Paydays that land on a weekend or bank holiday are shown on the business day before, which is what most employers do. Bank holidays follow the Federal Reserve calendar. Your employer's payroll calendar is the final word.