The day is from 12:00 p.m. to 11:59 p.m., while the night is from 12:00 a.m. to 11:59 a.m.
If it always seems to be day, your battery may have run dry, meaning time-based events can no longer occur.
Edit: The battery may not be dry, as I learned with sumwun's comment. It always appears to be daytime in Ruby, you have to check the clock in your house to determine whether it is night or day.
Hope I helped!