I looked this up. Let's read this in more context (this is from volume 4 of Encyclopaedia Biblica, by the way). Note that the quotation differs a little from what's actually said; the "i.e. Creation Sabbath" is not in the article, and it also changes "corresponding with changes in the moon's phases" to "corresponding with the changes in the moon's phases". The latter isn't a change in meaning, but it is odd it happens. Anyway, here's it with some more context, and I've helpfully bolded the quoted portions:
"Jensen is cautious and reserved on the question of a Babylonian origin of the Sabbath, which, however, Gunken (Schöpf. 14) and Jastrow (op. cit.) expressly affirm. The bridge which Gunkel fails to construct between the Babylonian atonement-Sabbath and the Hebrew rest-Sabbath, Jastrow endeavours to point out. He remarks that the Heb. Sabbathon does in fact, like the Bab. sabbatum, convey the idea of propitiation or appeasement of the divine anger, and he is of opinion that the Hebrew Sabbath was originally a Sabbathon–i.e., a day of propitiation and appeasement, marked by atoning rites. At this state of development it was celebrated at intervals of seven days, corresponding with changes in the moon's phases, and was identical in character with the four days in each month (7th, 14th, 21st, and 28th) that the Babylonians regarded as days which had to be converted into days of propitiation. There were also, however, sabbathon days, such as the New Year's Day, the Day of Atonement, the first and eighth days of the annual pilgrimage to the chief sanctuary.
The introduction, in consequence of profound changes in religious conceptions among the Hebrews, of the custom of celebrating the Sabbath every seventh day, irrespective of the relationship of the day to the moon's phases, led to a complete separation from the ancient view of the Sabbath, whilst the introduction, at a still later period, of the doctrine that the divine work of creation was completed in six days removed the Hebrew Sabbath still further from the point at which the development of the corresponding Babylonian institution ceased. Hence the position of the Sabbath in the Priestly Code. The field, however, is still open for further investigation."
As you hold up this as apparently authoritative enough that simply quoting from it is sufficient, do you agree with the related assertions your source makes that the Sabbath did not begin as a day of rest among the Hebrews, but was rather a holy day of propitiation among the Babylonians that the Hebrews took and changed into a rest day? Or that (after the Hebrews later on changed it from the Babylonian timing to instead be once every 7 days) they came up with the "creation week" in Genesis to justify it after the fact?
In fact, even if the above claims of the article was true (and note the article clearly identifies this as a theory, not hard fact), it actually eviscerates your argument. First, it explicitly states that the days were the 7th, 14th, 21st, and 28th, but you based your own claim on the premise it was the 8th, 15th, 22nd, and 29th. Second, it explicitly says that "the custom of celebrating the Sabbath every seventh day" predates "the introduction, at a still later period, of the doctrine that the divine work of creation was completed in six days." In other words, this shift occurred prior to the writing of the Book of Genesis. And the writing of the Book of Genesis was pretty clearly done prior to the time of the New Testament. In other words, your own source asserts that long prior to the New Testament period, Jews were having a Sabbath every 7 days, which would mean they were doing so in the New Testament.