(Gal 4:8) Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.
(Gal 4:9) But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage?
(Gal 4:10) Ye observe days, and months, and times, and years.
(Gal 4:11) I am afraid of you, lest I have bestowed upon you labor in vain.
Paul was writing to Gentile Christians who had converted from paganism and adopted a strict Jewish way of life – going from one pointless extreme to another. They were keeping Jewish law strictly, so the days they kept were not pagan holy days. If we read the preceding chapters, we will see that Paul is talking here of the same ritualistic trappings, only this time in Judaism. He mentions circumcision in chapter 2 and he mentions the observance of days in chapter 4. The Gentiles had fallen for the heresy preached by the pro-circumcision party, which included the observance of the Old Covenant holy days – see Gal 2. They had previously been slaves to a similar mentality under their pagan beliefs – obsession with ritualistic observance of days. Paul comments on this in verse 8, and then comments on their newly acquired bondage to elements of Judaism such as he mentions in verse 10 – days, months, seasons, years.
The principle he shows is that the keeping of days is meaningless – and since the Sabbath is a day, it makes sense to include it here. Without any remark to the contrary to show otherwise, it is not sound reasoning when Adventists assume that their special day is excluded from Paul’s teaching.
Also, the phrase days/months/seasons/years follows the biblical example, and is a type of phrase used for the holy days of the Old Covenant. For more on this, see the section on Col 2:16.