Olá, @ebgilding! Here’s a summary:
- De manhã
- De tarde = à tarde
- De noite = à noite
Basically, except when referring to mornings, the preposition de and the contraction à are generally interchangeable
Here’s a helpful explanation of how we also use certain other prepositions/contractions in reference to time: De manhã? Na manhã? Pela manhã? - Portuguese Questions / Grammar - Practice Portuguese