Learn Finnish: Perehdyn näiden kurssien materiaaliin viikonloppuna. - I will familiarize myself with the material of these courses on the weekend.

The verb perehtyä (to familiarize oneself with, to delve into) is a verb of committing or focusing. Just like keskittyä (to focus on), it requires its target to be in the illative case. You can physically think of it as diving 'into' the material.

Why does näiden kurssien come directly before materiaaliin?

The phrase näiden kurssien is in the plural genitive case, meaning 'of these courses'. In Finnish, a genitive modifier almost always sits immediately in front of the noun it modifies, which in this sentence is materiaaliin.

Why is the essive case used for viikonloppuna?

The essive case ending -na or -nä is the standard way to express 'when' an action happens for days of the week and specific recurring calendar periods. Just like you say maanantaina for 'on Monday', you use viikonloppuna for 'on the weekend'.

Why is there a d in perehdyn when the dictionary form is perehtyä?

This is a classic example of consonant gradation. The dictionary form perehtyä has a strong -t-. When you add the first-person ending -n, you close the final syllable (-dyn). A closed syllable triggers the weak grade, which softens the -t- into a -d-.