Here's why:
* Common nouns are general names for people, places, things, or ideas (e.g., dog, city, book, happiness).
* Proper nouns are specific names for individuals, places, or things (e.g., Fido, New York City, The Lord of the Rings).
"Tonight" refers to a specific point in time, making it similar to a proper noun. You wouldn't say "I'm going to the movies tonight" and then replace "tonight" with a general term like "evening".
Let me know if you'd like more examples!