God blessed the seventh day. When God blessed the animals and human beings He said "Be fruitful and multiply." Can this mean that God also said the same thing when God blessed the seventh day?
Also what does it mean that God has declared the seventh day holy?
"Bless" means in this context to give a benefit.
The Sabbath was made holy by God in the Law given at Mount Sinai. "You made known to them Your holy Sabbath, and commanded them precepts, statutes and laws, by the hand of Moses Your servant" (Nehemiah 9:14). It was made holy, or set apart for a special purpose, by God to be the day the Israelites worshiped God. "Observe the Sabbath day, to keep it holy, as the LORD your God commanded you" (Deuteronomy 5:12).