This has to mean that the proper way to find the holy festival of the Sabbath is to count from the new moon. Philo continues by saying that the Jewish nation keeps every seventh day holy after each interval of six days. This poses no problem at all to lunar sabbatarians as we too do what Philo did. Philo already gave us the proof that counting from the new moon was the proper way to find the holy festival of the Sabbath, therefore when he says the Jews keep every seventh day holy, he is speaking of every seventh day after each interval of six working days periods from new moon to new moon. This is the only logical way to understand this particular passage in Philo.

If Saturday keeper were to say that "some states keep the holy Sabbath only once a month counting on today's calendar, but we keep every seventh day after six workdays" would the only possible way to construe what you're saying is that your Holy Sabbath day was found in a different way than by the "calendar" or were both of you counting on today's calendar?
You just acknowledged that they kept the holy Sabbath day "only once" a month and they found it by today's calendar that Julius Caesar introduced, but that don't mean that you don't use the "same" calendar now does it? The same is true with Philo, they both counted from the new moon, but some were keeping it only once each month/moon. It is common knowledge most people use the calendar of today but when Philo made the very same statement, the weeks and holy Sabbath days were by the moon, that too was common knowledge.

When Philo made the statement that "some states keep the Holy Sabbath/festival only once a month/moon counting from the new moon" and follows up by saying they keep every Sabbath day after six days does not mean that he did not count for the holy Sabbath/festival in the same way that the people that kept it only once a month/moon did i.e. from the new moon.

The same thing applies to you if you said that some people keep the holy Sabbath/Saturday only once a month by the calendar but you keep every Sabbath after six workdays. (That doesn’t mean that you counted a different way or had a different calendar than they) I know a Baptist Church that has service every third Sunday counting on the today's calendar and I know another Baptist church that keeps every Sunday after six workdays are we to suppose that there are not counting by the same calendar? Philo used the word intervals and we know that there are approximately 12 monthly "intervals" each year and each month has four Sabbaths with "intervals" of six work days between each with a Sabbath at the end of each and Philo kept every one of them not only one per moon. Speaking of intervals, Philo in The Special Laws 1. (178) speaking of "intervals" or after six work day “lunar” intervals, Philo writes,

(178)…”there is one principle of reason by which the moon waxes and wanes in equal “intervals”, both as it increases and diminishes in illumination; the seven lambs because it receives the perfect shapes in periods of seven days— the half-moon in the first seven day period “after” its conjunction with the sun, full moon in the second; and when it makes its return again, the first is to half-moon, then it ceases at its conjunction with the sun.

If you would stop and think for a minute, how that Philo acknowledges that some states were finding the sacred holy Sabbath by counting from the new moon but was keeping it "only once" every moon. In reality Philo would have said "Some states keep the holy Sabbath only once a "moon" or "new moon", I don't believe he used the word "month" which proves that the weekly Sabbaths were by the moon, at least you will have to acknowledge that some people were keeping the Sabbaths by the moon, (not month) or they kept at least one of the holy Sabbaths each moon/month.

As for now allow me to continue in Philo’s writings: ALLEGORICAL INTERPRETATION, 1 IV (8) ”Again, the periodical changes of the moon, take place according to the number seven, that star having the greatest sympathy with the things on earth. And the changes which the moon works in the air, it perfects chiefly in accordance with its own configurations on each “seventh day”. At all events, all mortal things, as I have said before, drawing their more divine nature from the heaven, are moved in a manner which tends to their preservation in accordance with this number seven. … Accordingly, on the seventh day, God caused to rest from all his works which he had made."…

Had a Hebrew speaking Jew wrote this he would have said “it perfects chiefly in accordance with its own configurations on each “Sabbath” day” instead of each “seventh” day because elsewhere in his writings, Philo identifies that when he mentions the seventh day he is speaking of the Sabbath day. Above, he tells us that the moon perfects its own configurations on each seventh day. It was understood, that at the end each period of six work days there would be a weekly Sabbath. The Greek speaking Jews referred to the Sabbath as the seventh day or the sacred seventh day, while in the language of the Hebrews it was termed Shabbat, or the Sabbath.

Continuing on with Philo: THE DECALOGUE XXX (159) ”But to the seventh day of the week he has assigned the greatest festivals, those of the longest duration, at the periods of the equinox both vernal and autumnal and autumnal in each year; appointing two festivals for thse two epochs, each lasting seven days; the one which teakes place in the spring being for the perfection of what is being sown, and the one which falls in autumn being a feast of thanksgiving for the bringing home of all the fruits which the trees have produced”…