Learn Hardcore Greek: Ο ρυθμός της δουλειάς σήμερα είναι γρήγορος, αλλά προσπαθώ να είμαι ήρεμος. - The pace of work today is fast, but I try to be calm.

Ο is the masculine singular definite article (the). Greek nouns normally come with an article, and the article shows the noun’s gender, number, and case.
So Ο ρυθμός = the pace/rhythm (masc. singular, nominative).

Why is it της δουλειάς and not η δουλειά?

Because της δουλειάς is genitive: it means of the work / of work / at work depending on context.
Greek commonly uses NOUN + genitive to express relationships like the pace of the work.

η δουλειά = the work (nominative, subject form)
της δουλειάς = of the work (genitive, possession/association)

Is ρυθμός literally “rhythm”? Why is it used here?

Yes, ρυθμός can mean rhythm, but in everyday Greek it also commonly means pace/tempo.
So Ο ρυθμός της δουλειάς is a natural way to say the pace of work / the work pace.

Why is σήμερα placed after της δουλειάς? Could it go elsewhere?