《旧唐书·本纪·卷十三》
十一月八日，冬至日，皇上亲自在郊外圆丘祭祀昊天上帝。
十一月庚午，日南至，上亲祀昊天上帝于郊丘。
On the 8th day of the 11th month, the winter solstice, the emperor himself offered sacrifices to the God of Heaven at the circular mound outside the suburbs.