There is no teaching that says the Sabbath was ever changed from Saturday to Sunday. Sunday has always, since the resurrection of Jesus, been the day of assembly for worship. The first Christians worshipped on Sunday (Acts 20.7; 1 Cor. 16.2) in recognition of Jesus's resurrection (Jn. 20.1). By the mid 50s (20 years later), Sunday was still being kept by the Church for worship. In the time of Justin Martyr (AD 100-165), Christians were still calling Sunday "the Lord's Day," and still in commemoration of Jesus's resurrection.
But connecting their Sunday worship with the Sabbath has very little evidence to warrant the connection. The early church never connected Sunday with the Sabbath. Also, there seems to be no expectation that on "The Lord's Day" (Sunday) one is to rest from one's labors, as was the Jewish law in Ex. 20.8-11. Sunday worship for the early Church was celebration and learning, breaking bread, and praying, but not a rest from work. In the early Church, Saturday was their day of rest from work and Sunday was their day of worship of Jesus. They generally couldn't afford to take 2 days off from work, as we do. Sunday was full of activity just like any other day. Calling Sunday "the Sabbath" is a relatively new practice.
There is no evidence that I know of associating the practice of the early church to worship on Sunday with worshipping the sun god. There's an awful lot of false information on the Internet, even Facebook!
For a long time, people have tried very hard to align early Christianity was an obsession with astrology & the zodiac and even as a syncretistic blend of Roman mythology and Jesus's teachings. There's only shaky evidence for these claims, and they all come out in the wash.
Frankly speaking, God never commanded that Christian worship be on a particular day. We read that Sunday is when the NT Christians gathered for worship, but we cannot claim that they assembled every Sunday or that Sunday was the only day they assembled.
So, as far as the claim on Facebook: It's true that Saturday is the correct Sabbath day. It's not true that anyone going to church on Sunday is worshipping the sun god.
We can talk more if you wish.