Why was both the seventh day and the eighth day set apart as sacred under the Law?
Under the Law of Moses the eighth day was also set apart as sacred. (Lev 23:36) Why? Here's the truth. You may not like it. The seventh day was holy because that is when the 144,000 "little flock" of anointed ones receive their heavenly inheritance (after 6,000 years or 6 days) (Luke 12:32) They are instantly made perfect AT THE START of the seventh day, hence no work was to be done on the seventh sabbath day. (1 Cor 15:51-53) The rest of mankind (the "other sheep") do not “come to (perfect) life” until THE END of the seventh day. (Rev 20:5; John 10:16) That is the start of the eighth day, also a day of rest under the Law of Moses. This reveals that there are two hopes - heavenly and earthly. The heavenly "anointed" class receive their reward on the seventh day and the earthly class receive their reward on the eighth day. Hence, both days are holy.