The Vietnamese calendar (Vietnamese: âm lịch; chữ Hán: 陰曆) is a lunisolar calendar that is mostly based on the lunisolar Chinese calendar. As Vietnam's official calendar has been the Gregorian calendar since 1954,[1] the Vietnamese calendar is used mainly to observe lunisolar holidays and commemorations, such as Tết Nguyên Đán and Tết Trung Thu.