Obviously it departs from the plain reading of The Text of Revelation 13. Which says it is the Number of the Name of a Man (Anthropos).
Some people will try to deny it actually is a number by saying no other time does The Bible use Gemetria, it always spells out numbers phonetically. Problem is the Greeks texts all put a line over the Chi-Xi-Stigama which in the rules of Koine Greek tells us it's Gemetria. The Bible does it different here because the context tells us Gemetria is what it is about.
But for Seventh Day Adventists (who I think are right that there is no NT basis for replacing The Sabbath with a different day) everything revolves around The Sabbath issue which is why they name themselves that.
First of all, Sunday worship does not actually break The Sabbath law, even if you think we are still under the Law. The Sabbath Law can only be broken by what you do or don't do on The Sabbath. Nothing in the Sabbath commands make it a sin to, if you're able, also rest a different day. In modern America most people get both Saturday and Sunday off work. So even if some global Law DEMANDING Sunday observance was made as Adventists predict, it would not stop Torah observing Christians or Jews from doing what they do on The Sabbath.
The only way it could be even remotely possible to violate The Sabbath with what you do on a different day would be maybe on Friday. We see for example with The Manna that Israelites tended to do extra work on Friday to prepare for The Sabbath. But on the First Day of the Week the Sabbath is over, so it's the least likely to be a day you would even be worrying about the Sabbath issue.
Meanwhile nothing in Revelation 13 can be taken as pointing to Sunday. Nothing about the First Day of the Week, and nothing about Sun-Worship.
If you really want to twist the text of Revelation 13 to allude to a Day of the Week, the Sixth Day is what makes sense, 666 being a multiple of 6. And I've observed reasons before to thematically link this part of Revelation 13 to Genesis 2 and Adam's creation. And that the Gemetria of Iesous is 888 has been thematically linked to the Resurrection being on Sunday.
The Antichrist would presumably be taking titles of Christ for himself. One of those is The Last Adam. Gnosticism and Kabbalah have given the Last Adam concept their own special meanings.
Islam interestingly does call for weekly observance on Friday to commemorate the creation of Adam. Because Islam has actually canonized the Apocryphal legend that God ordered The Angels to worship Adam.
I remain highly skeptical of the Islamic Antichrist theory, but this is an interesting observation.Main article: Jumu'ahThe Quran acknowledges a six-part Creation period (32:4, 50:38) and the Biblical Sabbath as the seventh-day (yaum as-Sabt: 2:65, 4:47, 154, 7:163, 16:124), but Allah's mounting the throne after Creation is taken in contradistinction to Elohim's concluding and resting from his labors, and so Muslims replace Sabbath rest with jumu'ah (Arabic جمعة ). Also known as "Friday prayer", jumu'ah is a congregational prayer (salat) held every Friday (the Day of Assembly), just after midday, in place of the otherwise daily dhuhr prayer; it commemorates the creation of Adam on the sixth day, as a loving gathering of Adam's sons. The Quran states: "When the call is proclaimed to prayer on Friday, hasten earnestly to the Remembrance of Allah, and leave off business: That is best for you if ye but knew" (62:9). The next verse ("When the prayer is ended, then disperse in the land ...") leads many Muslims not to consider Friday a rest day, as in Indonesia, which regards the seventh-day Sabbath as unchanged; but many Muslim countries, such as Saudi Arabia, the United Arab Emirates and Bangladesh, do consider Friday a nonwork day, a holiday or a weekend; and other Muslim countries, likePakistan, count it as half a rest day (after the Friday prayer is over). Jumu'ah attendance is strictly incumbent upon all free adult males who are legal residents of the locality.
But of course a Friday reverence can be connected to Catholic and other heretical Christian beliefs via the completely unsupported by Scripture Friday Crucifixion tradition. Perhaps the counterfeit mortal wounding and healing of the Beast will follow the Catholic model.
The fact remains, the plain reading of Revelation is that it's a name not a day of the week.