Quick explanation:

If you want to be more precise, you may use terms such as early morning, morning, afternoon or evening.

Son las dos de la madrugada.
It's two o’clock in the early morning.
Son las siete de la mañana.
It's seven o’clock in the morning.
Son las cinco de la tarde.
It's five o’clock in the afternoon.
Son las ocho y media de la noche.
It's eight thirty in the evening.

Long Explanation:
Let's review more detailed time within the hour. We use the verb ser (to be) to say the time. Notice that when you say the time with one o'clock (singular), you begin with es. Let's look at some examples:
Es la una menos cuarto. - It's fifteen til one.
Es la una menos veinte - It's twenty til one.
Es la una. - It's one o’clock.
Es la una en punto. - It's one o’clock sharp.
Es la una y cuarto. - It's one fifteen.
Es la una y veinte. - It's one twenty.
Es la una y media. - It's one thirty.

When you say the time after two, three or later (plural), you begin with son.

Son las dos. - It's two o’clock.
Son las cinco. - It's five o’clock.
Son las cinco en punto. - It's five o’clock sharp.
Son las cinco y cuarto. - It's five fifteen.
Son las seis menos cuarto. - It's fifteen til six.
Son las tres menos cinco. - It's five til three.

For midnight or noon, you begin with es.
Es el mediodía - It's noon.
Es la medianoche - It's midnight.