In the United States, how customary is it for professors to not check their emails around Christmas? I hope I don't sound like I'm complaining, and I understand professors and academics are humans. My question is more of one of inquiry rather than venting. I have e-mailed a few professors for certain professional opportunities. This is not my first time doing so, and I am familiar with proper e-mail etiquette and how to write a message to minimize the chances of a glaring ignore. I have also received replies in the past. However, I have e-mailed a few professors about a week ago, and my inbox is crickets so far. On average, is it customary for academics to take a break from e-mailing during the Holiday break?
Yes, it's common. Many take a break from work entirely during this season. Some reduce their working hours to spend time with family, and only answer urgent email. Some may be traveling and have limited or no access to email. Some (like me) may be swamped with grading and other end-of-semester duties and are focusing on those at the moment.

If you haven't heard back by mid-January, try a polite follow-up then.