Dawn is a synonym for break of day. It is the time before sunrise when it is possible to see light in the sky just before the sun reaches the horizon.
Ex: Every Saturday morning, my sister and I wake up at dawn to go for a nice, long run around the park. It is a great way to spend our morning.
Ex: Nikki starts her yoga practice at dawn every morning in the front garden. There is just enough light for her to see what she's doing and enough warmth in the air for it to be comfortable.
The phrase break of dawn is often used to describe the time when the first break of light can be seen in the sky. Because the light is seen before sunrise, break of dawn is very early in the morning. It could also be a synonym for daybreak.
Ex: When living on a farm, you can usually hear the rooster crow at the break of dawn. It is as if he wants to let everyone know there is sunlight in the sky.
Ex: The newspaper delivery boy starts his work at the break of dawn every day. He waits until he sees light in the sky before getting on his bike to deliver papers.
The moment after dusk when the last bit of sunlight is gone from the sky is nightfall. Nightfall occurs after sunset and marks the official beginning of night.
Ex: It's very interesting to see how the moon starts to shine a little brighter right after nightfall. It is almost like the moon waits for the sunlight to disappear for it to shine.
Ex: Every night, my friend Mike and I wait until nightfall to watch the stars from our backyard. Once the sky is completely dark, you can see all the constellations in the sky.
Twilight is the word used to describe the soft light that appears in the sky after the sun sets. Of course, dusk is the darkest period of twilight because that is when there is the least amount of light remaining.
Ex: If you are ever lost in the woods after sunset, use the twilight to help find your way to shelter. You have to make good use of the time left before the sky darkens completely.
Ex: Many sailors say they prefer to take their boats on the water after the sun goes down because the twilight creates more beautiful shadows on the water.