What is the Center of the Law?
Amazing what you'll see when you look at the 10 Commandments as recorded in Exodus chapter 20. Here is the complete KJV text:

20:3 Thou shalt have no other gods before me.
20:4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.
20:5 Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
20:6 And shewing mercy unto thousands of them that love me, and keep my commandments.
20:7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
20:8 Remember the sabbath day, to keep it holy.
20:9 Six days shalt thou labour, and do all thy work:
20:10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
20:12 Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
20:13 Thou shalt not kill.
20:14 Thou shalt not commit adultery.
20:15 Thou shalt not steal.
20:16 Thou shalt not bear false witness against thy neighbour.
20:17 Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's.

Fifteen verses covering 10 Commandments.
The middle verse is verse 10.
There are 7 verses before verse 10, and 7 verses following it.

There are 297 words comprising the 10 Commandments.
The middle word is the 149th word.
There are 148 words before the middle word, and 148 words following it.

The first 7 words of the middle verse are:
"But the seventh day is the sabbath"
The 149th word, the middle word of the 10 Commandments is found within this phrase taken from the middle verse which is located within the "central" fourth commandment. The middle word is "is", the equality function of the central formula:

seventh day  = the sabbath


And the Center of the Sabbath?

We're not through yet.

Sabbath in Hebrew is "shabath", composed of three characters:
Sh - ab - ath

ab = Father  [Strongs entry number 2]

ath = Sign, a portent [Strongs entry number 852, from 226 'awth']

awth = Signal, omen, monument, mark [Strongs 226]

"Sign of the Father", "Mark of the Father", "Memorial, monument to the Father"

In English, the core of the word "Sabbath" is "abba":

S - abba -th

Paul states that we cry "Abba, Father" in Galatians 4:6

In Revelation 7 the servants of God wait to be sealed in their foreheads, verse 3.

In Revelation 14 the 144,000 have the Father's name written in their foreheads, verse 1.

Thus, the Sabbath is the seal of the Father, chosen by His servants by their mental consent to His authority as Creator.