The time between when you put your application in the mail to when you receive acknowledgement of receipt (AOR1) is probably the most stressful times of the whole process. Until it's been 7 or 8 weeks, I wouldn't worry.
They are probably behind due to 3 statutory holidays (Dec 25, 26, Jan1) and I'd also guess they weren't at full staffing levels due to their staff wanting to take holidays in December.
In a few more week's time if you still haven't heard anything, you can call the call centre at 1-888-242-2100 (it may take several attempts to get in an "on hold" queue) to find out if indeed it was received but you failed to receive a notification.
The other possibility is that if your application was in some way incomplete, they will return it without any notification. Applicants who have had their applications returned usually find it in their mailboxes about 2-4 weeks after the time the AOR1 would have come. I don't mean to add to your anxiety.
I hope all is well and you receive your AOR1 any day now

Most likely it was just a delay due to staffing levels in December.