Por
is the preposition used to refer to a general part of the day
Por la mañana, por la tarde
It can be used with other nouns that determine time
Por Navidad. Por agosto.
The preposition en
can be used to form locuciones temporales
Denota en qué lugar, tiempo o modo se realiza lo expresado por el verbo a que se refiere
- with the name of months: En agosto.
- names of seasons: En primavera.
- with years: En 1984.
- certain nouns: Los Reyes Magos vienen en Navidad.
The preposition a
can be used to refer to hours of the day
a las diez de la mañana (or de 10am a 7pm)
It seems that most Spaniards would say "por la mañana" and some Latin Americans accept "en la mañana". There are even some regionalisms that would accept the use of a
Nos vemos a la tarde
But I don't think I have ever heard this with "mañana" ("a la mañana") and means something closer to "next evening" than "every evening".
My suggestion would be to user por
to describe a time frame of the day by name.