But,but,but they were GENTILES not Jews,so how did they know how to keep the Sabbath? 
we're not told.
isn't that odd?
we're not told.....
just seems like Paul is really ticked off that they were listening to the Judaizers.
Galatians 4
9But now that you have come to know God, or rather to be known by God, how is it that you turn back again to the weak and worthless elemental things, to which you desire to be enslaved all over again? 10
You observe days and months and seasons and years. 11I fear for you, that perhaps I have labored over you in vain.
...
Verse 10. - Ye observe days, and months, and times, and years (ἡμέρας παρατηρεῖσθε, καὶ μῆνας καὶ καιρούς καὶ ἐνιαυτούς); days ye are intent on observing, and months, and seasons, and years. In the compound verb παρατηρεῖν, the prepositional prefix, which often denotes "amiss," seems rather, from the sense of "at one's side," to give the verb the shade of close, intent observation. This may be shown by the circumstances to be of an insidious character; thus the active παρατηρεῖν in Mark 3:2; Luke 6:7; Luke 14:1; Acts 9:24, and the middle παρατηροῦμαι, with no apparent difference of sense, in Luke 20:20.
Josephus uses the verb of "keeping the sabbath days" ('Ant.,' 3:05, 8), and the noun παρατήρησις τῶν νομίμων, for "observance of the things which are according to the laws" ('Ant.,' 8:03, 9). The accumulation of nouns with the reiterated "and," furnishing another example of the δεινότης of St. Paul's style, betokens a scornfully impatient mimesis.
These reactionaries were full of festival-observing pedantry - "days," "new moons," "festivals," "holy years," being always on their lips. The meaning of the first three of the nouns is partially suggested by Colossians 2:16, "Let no man judge you... in respect of a feast day, or a new moon, or a sabbath day (ἑορτῆς νουμηνίας, σαββάτων);" in which passage, we may observe, there is a similar tone of half-mocking mimesis; where the same ideas are apparently presented, but in a reverse order. Comp. also 2 Chronicles 8:13, Offering according to the commandment of Moses, on the sabbaths, and on the new moons, and on the solemn feasts, three times in the year, even in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles."
The "days," then, in the present passage, we may suppose, are the sabbath days, together perhaps with the two fast days every week which the Jewish tradition prescribed (Luke 18:12). The "months" point to the new moons, the observance of which might occasion to these Gentiles considerable scope for discussion in adjusting themselves to the Jewish calendar, different no doubt from the calendar they had been hitherto used to.
The "seasons" would be the annual festivals and fasts of the Jews, not only the three prescribed by the Levitical Law, but also certain others added by tradition, as the Feasts of Purim and of Dedication. So far we appear to be on tolerably sure ground.
The fourth item, "years," may refer either to the sabbatical year (Leviticus 25:2-7), which at any rate latterly the Jews had got to pay much attention to (1 Macc. 6:49, 53; Josephus, 'Ant.,' 14:10, 6; also 14:16, 2; Tacitus, 'Hist.,' 5:4); or possibly the jubilee years, one such fiftieth year, it might be, falling about this time due. Bengel ('Gnomon') supposes that a sabbatical year might be being held A.D. , to which date he assigns this Epistle; while Wieseler ('Chronicles Synops.,' p. 204, etc., referred to by Bishop Lightfoot) offers a similar conjecture for the year A.D. autumn to A.D. autumn. Very striking is the impatience which the apostle manifests in overhearing as it were the
eager discussions occupying the attention of these foolish Galatian Judaizers. - PULPIT
