In this chapter, Moses provides instructions for the celebration of the three core festivals, the Passover, Feast of Weeks and Feast of Booths.
We have seen these feasts described over and over and over, and now we read about them again. Ex 23 was the first declaration of the covenant, Ex 34 was the second declaration (after the sin of the golden calf), and Lev 23 was a consolidated set of instructions for the priests related to the festivals. Deuteronomy is, in a sense, a third declaration of the covenant. This chapter's descriptions of the feasts is one part of that covenant.
On previous occasions, I described these feasts as important social and religious events, but by now it should be evident they are also important to the covenant. Important in what way? That is a bit harder to explain, and anyway I have written about it a lot before. There are connections to the agricultural cycle and to their exodus from Egypt. The Passover is an obvious memorial to the exodus from Egypt and deliverance from slavery.
The latter two feasts are harder to place in the covenantal language, but since they occur during the harvest I think they represent the occupation of the promised land. In this view, the covenant has two big parts: freedom out of slavery, and passage into the promised land. The harvest cycle can be viewed as a metaphor for this, as the Passover occurs during the spring planting, while the feasts of Weeks and Booths occur towards the end during the harvest.
In a similar fashion, we can imagine freedom from slavery as being like sowing a crop, because it means nothing to be freed from slavery if you are only sent into a desert to die. The whole purpose of the Exodus was to free the Israelites in order to bring them in to the land of promise (Ex 3:8), which is likened to reaping a crop.
So if that's how the three feasts fit into the covenantal narrative, why do the Israelites celebrate them? As an act of memorial, to remember what the LORD has done for them.
Moving on, there are obvious differences between here and the Exodus accounts. For one, the Exodus accounts are remarkably terse. This chapter is quite expansive by comparison. This chapter also contains some uniquely Deuteronomic language, such as "the place which the LORD will choose", a term I have discussed before. This chapter also emphasizes the "Levite, stranger, orphan, widow" formulation which does not occur anywhere outside of Deuteronomy.
In similar terms, this chapter also lays out one's expanded household as "you, sons, daughters, male servants, female servants" and then an outer tier of dependents, the vulnerable classes (listed above, Levite, stranger, etc). We can compare this to Ex 20:10 which presents a very similar listing, except instead of the "Levite, stranger, orphan widow" it lists the "sojourner who stays with you." This is most likely a reference to hired workmen, which is not a vulnerable class but rather a loosely associated part of the extended family.
Nevertheless, the core rhetoric is the same, with an emphasis on appearing before the LORD three times a year and the general description of the feasts is largely similar to before.
The very end of the chapter is about the appointment of judges which does not relate to the feasts at all. It fits in more closely with the next chapter concerning the administration of justice and the future kingdom.
Lastly, this chapter thematically relates to the fourth commandment, to honor the Sabbath. The Sabbath and the festivals are similar because they are all a time to cease from labor, to gather together and to reflect upon the LORD. In Lev 23, the Sabbath is listed together with the feasts as "holy convocations".