One of the rules we learned in school was “Day is masculine, but its parts are feminine.” That has been a huge help in remembering the gender of día.
However, it is somewhat humorous to still hear native Spanish speakers say, “Buenas Días.”
Mañana
This can mean tomorrow or morning. Most of the time when it means morning it will be in the phrase “la mañana.”
Tarde / Noche
These are afternoon and night. It is somewhat ambiguous as to when tarde ends and noche begins. This is probably a very regional distinction; however, where we live, 8:00 in the evening is somewhat the separation time. When it gets dark at 6:00 in the afternoon, it is hard to think that the night has not started. Or in the summer it can be well after 9:00 before the sun drops below the horizon. The cut off from one to the other is very flexible.
Remember the rule “Día is masculine, but its parts are feminine.”
[…] mentioned some other parts of the day previously. The rule that says “The day is masculine, but its parts are feminine,” […]