What were the practices observance of which was abhorred by St Paul in Gal 4:9-10?

It wasn't the practices per se, it was the goal the people were attempting to get to of the practices that Paul abhorred. Galatians is full of the contrast between the Mosaic Law and Grace in Christ.

To understand Paul's point, we need to understand the point of the Mosaic Law.

And it [the whole of the Mosaic Law] shall be our righteousness, if we observe to do all these commandments before the LORD our God, as he hath commanded us. Deut 6:25

Yes, Christ came to fulfill the Law, not destroy it. And in its fulfilling, Christ became our righteousness by grace through faith.

And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: Phil 3:9

Galatians is full of the contrast between grace and works. Paul's prime example is the two women metaphor between Agar/Sinai/Law and Isaac/Zion/Grace. Grace saves, doing what the Law could not.

Some of those in Galatia failed to understand this.

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 4:9

The word "beggarly" is ptochos, to beg, hide, or cower. See Vines, Strongs, etc.

Ye observe days, and months, and times, and years. Gal 4:10

The word "observe" is paratereo, "translated "ye observe" in Gal 4:10, where the Middle Voice suggests that their religious observance of days, etc. was not from disinterested motives, but with a view to their own advantage." Vines here.

In other words, they were carefully observing certain times with a view not to honor God, but as beggars to perchance be saved from their sins.

Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need. Heb 4:16

Christians observe Easter, Christmas, and other times. Why? To cower because they doubt their salvation or to boldly enter the throne of grace?

The practices being observed by Galatian Christians, which Paul so abhorred he wrote in the strongest terms to dissuade them from doing, were those things back then causing them to return to bondage. That is what the end of verse 9 states.

This means that they had stopped doing some things after Christ had set them free, but now they were going back to those practices, not realising that that was jeopardizing their freedom in Christ, getting them back into bondage.

This is about spiritual bondage. From the start of chapter 3 Paul asks the Galatians (whom he calls 'foolish') who has bewitched them into no longer obeying the truth. He says they had begun in the Spirit (being made spiritual sons of God, through faith in Christ), but now they were seeking to be made perfect by the flesh (verse 3). So, there's what Paul is distressed about - they are now seeking to be "made perfect by the flesh" instead of by the Spirit. Some people were persuading them that they still needed to keep the (Mosaic) law to be justified. Paul categorically told them "that no man is justified by the law in the sight of God, it is evident, for 'The just shall live by faith'." (verse 11) He pointed out that Abraham was accounted righteous by faith - before the law was given.

When the law was given later, it was to show God's people that they would be cursed if they did not keep everything in it. Now Christ had come and redeemed them from the curse of the law (verse 13),

"that the blessing of Abraham might come on the Gentiles through Jesus Christ: that we might receive the promise of the Spirit through faith." Galatians 3:14 A.V.

Paul then reminded them in chapter 4 that they had been in bondage under the law. But now the time had come to receive the adoption as God's son, proof of this spiritual transformation being the giving of the Spirit to them, so that they could now cry to God, "Abba! Father!" (verses 6 & 7).

But now they were turning back to some observances that they had been freed from. In their case, it happened to be religious observance of certain days, months, times and years. He does not mention circumcision, so it wasn't that in their case (though it had been with some others who would be bound to keep the law if they became circumcised.) No, they were binding themselves to a religious calendar of events. He does not spell out the details of those events because they would be well known to both him and them.

To apply this practically, to ourselves in this day and age, a letter rebuking modern-day Christians for returning to "weak and miserable elements" could be anything that makes us trust in something that we used to do that we thought was a legal requirement for getting (or staying) right with God. There are, for example, lots of superstitions some religious people think are important to observe. Or there are particular 'holy days' they must devote themselves to keeping the right way to keep 'on course' with God. Compared with the glorious spiritual freedom in Christ Christians are given freely, those would be "weak and miserable elements". That is why Paul goes on to explain the allegory of the free woman, Sarah, and the bond-woman, Hagar. Christians are children of the free woman (verse 31), so why on earth would they want to turn to bondage again? Why on earth would they start doing worldly 'spiritual' things that were in opposition to the spiritual miracle of grace that had made them free sons of God, in Christ?

So, the answer to the question is that they were those things that would chain them to the law, stopping them from walking in the freedom of faith in Christ. In our case it might be something different. We are to examine ourselves to make sure we never get into a worldly system of doing (or believing) things that undermine or outrightly attack our freedom in Christ. This was purchased for us at great cost, though freely given to us. Yet once set free, we should devote, and even be prepared to give, our lives for the One who liberated us - Christ.