Why did God bless the seventh day and make it Holy?

This is because, in the christian religion, it is said that God created the Earth in six days and rested on the seventh. Just as he worked for six days and rested on the seventh day (sabbath), man should rest on the seventh day and give worship to the creator. The day of worship was changed by the Roman Catholics from Saturday to Sunday for some unknown reason. Due to this change, the Catholic Denomination of Christianity was created.

Hope this answer helped.