Learn German: Seit Freitag regnet es. - It has been raining since Friday.

In German, if an action started in the past and is still happening, you use the present tense with seit. English uses the present perfect continuous ("has been raining"), but German simply uses regnet (rains).

Why is the word order regnet es instead of es regnet?

This is the "verb second" rule. Because the time phrase Seit Freitag takes up the first position in the sentence, the verb regnet must come second. The subject es is then pushed to the third position.

Could I use von instead of seit to say "from Friday"?

No, seit is the exact word you need when something started in the past and is still happening now (just like "since" in English). You generally use von when you are talking about a specific start and end point, usually paired with bis (like von Freitag bis Sonntag, meaning "from Friday to Sunday").

The lesson says seit requires the dative case. Where is the dative case in seit Freitag?

When you use seit with general days of the week, months, or years, you don't need to use an article (like dem). You just use the bare noun. Because there's no article and Freitag doesn't take a special dative ending here, the case is essentially "invisible"!