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.
Leaving aside the timing of your request, if you were not my student and cold emailed me about a "professional opportunity," I would view it as academic spam and would put it in the lowest priority queue, right after re-upping my free professional journal subscriptions.
It might just be your phrasing, but from my perspective as faculty, it sounds more like opportunity cost than opportunity.
While we can't guess for the particular professors you're dealing with (and some professors never really take any time off), it's quite common for people in the United States to take significant time off around the holidays. The more responsible ones will set a vacation message letting you know when they'll be back, but really, you shouldn't really expect to hear anything from anyone until significantly after New Years.
In addition to the excellent answers given already, you should also consider that even those professors that do answer emails during the holidays will usually do so based on perceived priority - and I doubt that your cold-call "professional opportunity" email is important enough to many professors to answer between visits to relatives and stuffing them with whatever their traditional holiday food is.
You should try resending again when the holiday period is over.
It's easy for stuff to fall through the cracks during the Christmas-->New Years vacation time, at least in countries for which this is the central winter holiday period. Don't take it personally.
I would find out, for each professor, the academic calendar where they work (this is always easy to find online). I'd wait until the middle of the first scheduled week of classes, and send them a polite follow-up email. If they don't respond within a week, then they are probably not interested.
First, you are not entitled to the professors' time in matters of your personal professional development. You asking for a favour from them here, and, even if you frame it as a matter of etiquette, it is something you want from them and thus, you should consider yourself more on the side of the obligation then them.
Second, concerning permanent reachability per email: I do not expect my research fellows, PhDs, MSc or BSc project students to respond to emails etc. during weekends or vacation; my arrangement with them is done in advance in such a way that contact during their rest/break time becomes necessary only in the rarest emergency; in which case I escalate to phone contact.
You may want to consider the possibility that the professors you are contacting may want to enjoy the same privilege. Clearly, as your search for a professional opportunity, this was not unexpected and thus in no state of extreme urgency, but could have been planned ahead of time by you to avoid the holiday period.
To come back to my illustration, the test is that if you feel it may be intrusive to ring the professor up at 23:00 on Saturday night or during the morning of Boxing Day (which you likely will), you should also not expect an email from them during this time (at least I hope it for the sake of their family).
That being said, some people do not respond during their absence, but then empty their mailbox after they are back to work; I would consider this equally disrespectful on the flip-side of your problem. For such people (or simply forgetful ones), a polite reminder, shortly after the vacation ends, is perfectly in place.