Learn Spanish (Spain): El vuelo sale a las siete en punto.

We use son las or es la to state what time it is right now. But when you want to say that an event happens at a certain time, you do not use the verb ser. Instead, you use a (at). Because the flight leaves at seven, we say a las siete.

Why do we use the feminine plural las before the number?

In Spanish, hours are always feminine because they refer to the hidden word hora (hour). Because seven is a number greater than one, we use the plural las. If the flight left at one, we would say a la una.

What does the phrase en punto mean here?

It literally translates to "in point". This is the Spanish way of saying "on the dot", "sharp", or "exactly". You use it to specify that the time is exactly on the hour, with no extra minutes.

Which verb does sale come from?

It comes from the verb salir, which means "to leave" or "to go out". Since the flight is a single thing, we use the normal "it" form of the verb, which is sale. Just remember that the "I" form of this verb is a little irregular: yo salgo (I leave).