The Amplified is of course a commentary. Most modern commentators are eager to group God's holy days and their observance into this category. No surprise there. That being said, Judaism, by the time of Christ, had corrupted the true religion given by God. So some of the non-scriptural, perhaps pagan elements, of Judaism could well have been referred to.
Lev 23:2 Speak unto the children of Israel, and say unto them, Concerning the feasts of the LORD, which ye shall proclaim to be holy convocations, even these are my feasts.
Lev 23:26 And the LORD spoke unto Moses, saying, Lev 23:27 Also on the tenth day of this seventh month there shall be a day of atonement: it shall be a holy convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the LORD.
Lev 23:28 And ye shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the LORD your God.
Lev 23:29 For whatsoever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people.
Lev 23:30 And whatsoever soul it be that doeth any work in that same day, the same soul will I destroy from among his people.
Lev 23:31 Ye shall do no manner of work: it shall be a statute forever throughout your generations in all your dwellings.
Lev 23:32 It shall be unto you a sabbath of rest, and ye shall afflict your souls: in the ninth day of the month at even, from even unto even, shall ye celebrate your sabbath.