Sol Invictus commemorated the Solstice, which as the article states "had long been sacred to Aryan peoples of many nations; Aurelian chose a specific date and made it official. He did this in hopes of establishing a new religious unity among Roman citizens."

Aurelian wasn't Christian. He primarily followed the traditional Roman polytheistic religion but prominently elevated the cult of Sol Invictus, the "Unconquered Sun," as the empire's dominant state deity during his reign (270-275 CE).