"It is for freedom that Christ has set us free. Stand firm then and do not let yourselves be burdened again by a yoke of slavery." (Galatians 5:1, NIV)
Paul is worried about these people, that they could lose everything they have gained through their faith in Jesus. The stakes are incredibly high. These free people want to act like slaves. Paul will have none of it.
So what's going on? Think of the Law of Moses as a pair of stabilisers on a bike*. They stop you from falling over, up to a point, but they don't teach you the skills you really need to cycle properly. You don't see cyclists using them at the Olympics. If you want to cycle round sharp bends, quickly, or on uneven ground stabilisers will do more harm than good.
These people were carefully keeping special days, months, seasons and years because the Law of Moses told them to. The word that Paul uses suggests that this was an obsession. But that's not the point of believing in Jesus; the point is that faith should express itself in love.
Freedom in Christ is not about obeying the rules. To be Christ's followers we have to remove the stabilisers.
More on Sunday ...
* I'm grateful to Richard Rohr for this idea.