Learn Dutch: Het regent zelden in de zomer. - It rarely rains in the summer.

Yes! Zelden means "rarely" or "seldom". It behaves exactly like vaak (often) or altijd (always), and normally sits right after the conjugated verb (regent) in a standard sentence.

Could I put zelden at the very beginning of the sentence?

You can, but it changes the word order. If you want to emphasize how rarely it rains, you can start with Zelden. Because Dutch sentences always put the verb in the second position, the verb and subject must swap places: Zelden regent het in de zomer.

Why is it in de zomer when in English we can just say "in summer"?

In Dutch, you must use the definite article (de or het) when talking about a season. All four seasons take de: in de zomer (summer), in de herfst (autumn), in de winter (winter), and in de lente (spring).

Why is the subject het here?

Just like in English, Dutch uses the "dummy subject" het (it) for weather conditions, because there isn't a specific person or object doing the raining. The infinitive verb is regenen (to rain).