What is ( really ) so important about The Day That We Worship? And, should we keep The Sabbath, or Sunday?

Saturday, or Sunday - who ( quite honestly ) cares?

One of the most Frequently Asked Questions that I've ever heard is: " What Is Really So Important About The Actual Day That We Worship " - after all, as so many of us currently believe, ' There Can Be Nothing More Arbitrary Than Just Picking One Day From Seven To Now Set Aside For Our Worship '.

And, in fact, since we're always utilizing the word: " Arbitrary " - in reference to All Of Those Commandments, as given by God - let's, now, just examine it for the purpose of spiritual clarity.

According to MerriamWebster.com, the word Arbitrary is clearly defined as follows:

1. Existing or coming about seemingly at random; or by chance; or as a capricious and unreasonable act of will. 
2. Based upon or determined by individual preference or convenience; rather than by necessity or the intrinsic nature of something.
3. Not restrained or limited in the exercise of power; ruling by absolute authority.
4. Marked by, or resulting from, the unrestrained and often tyrannical exercise of power.
5. Depending upon individual discretion ( as of a judge ) and not fixed by law.

And, subsequently, we can very clearly see that ( contrary to popular opinion ) ' In Fact Nothing Could Be Less Arbitrary Than The True Biblical Sabbath Day '... since, it stands apart from every single definition of Arbitrary.