What is the meaning of "a sabbath" day?

When you schedule an appointment, with a chiropracter, a date in the calendar, and a time of the 24 hour day is specified.

So, with our Heavenly Father, He has specified a date in His Calendar, and the time apportioned of the day is the time of the daylight period.

If you use a different system of calendation apart from His Calendation, then how do you suppose you can spend the Sabbath with Him?

If you come at the wrong time, then you cannot expect to receive any greater blessing, than any of the other six working days.

But if you meet with Him, on the correct day He calls the seventh day, and especially The Qudesh, the New Moon day; then you can hope for the richest of all blessings mortal man can hope for.

This is why Lucifer changed the calendar as prophesied in:

Daniel 7:25
25 And he shall speak [great] words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.  (change the calendar & the fourth commandment giving directions of calendation)

Today's "altared" fourth commandment only specifies the 7th day of the week.

The original "fourth" command, has a starting point for calendation.

"Mark even day, the New Moon, to consecrate it." then

"Six days shalt thou labor and do all thy work" then

"And the seventh day is a Sabbath unto YHVH thy Elohim" and

"In it thou shalt not do any work ..."

This is what it means

Isaiah 58:12
12 And [they that shall be] of thee shall build the old waste places: thou shalt raise up the foundations of many generations; and thou shalt be called, The repairer of the breach, The restorer of paths to dwell in.

Prophets and Kings page 678
In the time of the end every divine institution is to be restored. The breach made in the law at the time the Sabbath was changed by man, is to be repaired.