What day has man created? Hasn't God created them all?
What?? Still not going to answer? :-)
The Lord Jesus Christ created specific days to honor him. They're listed in the bible. He observed them. His disciples observed them. The first Christians observed them.
The only reason EVERYONE doesn't observe them is because the Roman empire HATED jews because they dared to revolt against Rome three times...at least.
So Rome based Christians took a lot of heat for looking like the cursed jews....so they abandoned biblical days and made up their own and adopted already existing ones so they could fit into society more easily.