Learn Farsi: اوایل بهار، دانشجویان نجوم آسمان را رصد می‌کنند. - In early spring, astronomy students observe the sky.

avayel is the Arabic broken plural of avval (first/beginning). In Farsi, this plural form is grammaticalized to refer to a span or stretch of time rather than a single point. So avayel-e bahar literally means "the early days/parts of spring." You always connect it to the season or time period using the ezâfe (-e).

Why isn't there a preposition like dar (در) before avayel-e bahar to mean "in early spring"?

In Farsi, when a time phrase acts as an adverb at the beginning of a sentence, the preposition dar is frequently omitted. Saying dar avayel-e bahar is also completely correct, but dropping the preposition sounds very natural and fluid in this context.

Is rasad mikonand (رصد می‌کنند) the common way to say "they observe" in everyday conversation?

No, the compound verb rasad kardan is very specific to scientific or systematic observation, most commonly in astronomy (like looking through a telescope) or surveillance. For casually observing or watching something, you would use negah kardan (to look) or tamasha kardan (to watch).

Why is the plural daneshjuyan (دانشجویان) used here instead of the more standard daneshju-ha (دانشجوها)?

While -ha is the standard plural marker for almost everything in spoken Farsi, -an (which becomes -yan after a vowel) is the formal plural suffix for living beings. Because this sentence is about an academic topic and sounds slightly formal, fits the register perfectly.