Learn Spanish (Latin America): Mi viaje a Perú es en febrero o marzo.

The preposition a means "to" and is used to show your destination. Since a trip (viaje) involves traveling to a place, we use a Perú. If you said viaje en Perú, it would mean you are already there and taking a trip entirely inside the country.

Why do we use es instead of está here?

You might remember that the verb estar is used for physical locations, but here we are talking about when an event (the trip) is scheduled. In Spanish, you always use the verb ser (which gives us es) to tell the time or date that an event takes place.