Nothing revealing! A tshirt, jeans or skirt is ikay

Just show up. You will love it! Have a great trip!

I cannot add to the previous answer. I believe they post mass times on the internet.

Some parts of the city centre are quite safe, and this is one of them, as many governmental offices are situated in that area. But it depends where will you be staying as you may need to travel from your hotel for going to the Zocalo. You should take a uber for going to the Zocalo or the closet street near that place. Sunrise is around 6 and 7 am Have a great trip. Óscar

Sí. Uber es la forma más rápida y segura de moverse por la ciudad. Probablemente encuentre más información en el foro de viajes de la Ciudad de México.

No, my friend, all masses seem to be in Spanish… yet, don't miss the experience: see, smell, listen, feel the crowd, enjoy the Cathedral in such a formal moment. It's an extraordinary experience, regardless of the language. You won't regret it –and, if you are a catholic, you know the rite, so you can follow it quite easy. If you are not, just be there, respectfully, and live the experience. Ah, and welcome to Mexico!

Si la entrada es gratuita, solamente si quieres entrar a las catacumbas (parte inferior de la catedral) hay horarios establecidos y tambien ver que no haya trabajos de reparacion y mantenimiento. Saludos

Yes, there are tours every 45 minutes or so, and you can find the info at the entrance of the cathedral, either on a stand or on a table. As to the dress code, remember: you are visiting a city, not the beach, and you are entering a church, so do the polite thing is to wear long pants and shirts that cover your arms and any cleavage.