To find out what the mark of the beast is, you need to know what the mark of YHVH is:
Exo 31:12 And יהוה spoke to Mosheh, saying,
Exo 31:13 “And you, speak to the children of Yisra’ĕl, saying, ‘My Sabbaths you are to guard, by all means, for it is a sign between Me and you throughout your generations, to know that I, יהוה, am setting you apart. Footnote: 1The only sign of יהוה setting us apart, the only sign of the everlasting covenant, is His Sabbaths, one of them being the seventh day Sabbath. This is repeated in Ezek. 20:12 & 20.
Exo 31:14 ‘And you shall guard the Sabbath, for it is set-apart to you. Everyone who profanes it shall certainly be put to death, for anyone who does work on it, that being shall be cut off from among his people.
Exo 31:15 ‘Six days work is done, and on the seventh is a Sabbath of rest, set-apart to יהוה. Everyone doing work on the Sabbath day shall certainly be put to death.
Exo 31:16 ‘And the children of Yisra’ĕl shall guard the Sabbath, to observe the Sabbath throughout their generations as an everlasting covenant.
Exo 31:17 ‘Between Me and the children of Yisra’ĕl it is a sign forever. For in six days יהוה made the heavens and the earth, and on the seventh day He rested and was refreshed.’ ”
We see that the 7th day sabbath is a sign of who His people are because they honor the Creator on the ordained day. They also keep the Lev 23 feasts which are also known as sabbaths and show His plan of salvation.
The word sign can also have these definitions:
Probably from H225 (in the sense of appearing); a signal (literally or figuratively), as a flag, beacon, monument, omen, prodigy, evidence, etc.: - mark, miracle, (en-) sign, token.
So the sabbath is a "Mark" upon His people. Also, keeping all the words of Elohim is a sign (which I could post many verses about this). It shows who truly believes in Him and follows Him.
The mark of YHVH = Sabbath and obedience
The mark of the beast = disobedience and false sabbaths