When I recently inquired about the status of my paper which is currently in the "under review" status, the editorial assistant informed me that they are still awaiting referee reports, and the Editor is expecting that the review process will be completed at the end of the January. Its been a year I have submitted, and I am a bit startled that the editor has given a specific time of completion, rather than just say that it's still pending. Could there be any specific reason? Do reviews happen more in the holiday season generally? Assuming the reviewers have ample leisure time to study the paper? Does this indicate anything about the incoming decision?
Reviewers set their own schedule subject, more or less, to deadlines set by the editor. Some might use holidays to work on reviews. Others might rather avoid anything that looks like work. The "end of January" may be a guess or may be more firm, depending on the deadline request sent to reviewers. Perhaps a reviewer has made a "promise" to be done by then.
And what is "expected" may not come to pass since reviewers aren't employees, but volunteers.
There're several questions in the OP:
I am a bit startled that the editor has given a specific time of completion, rather than just say that it's still pending. Could there be any specific reason?
The most likely reason is because they invited a reviewer requesting a review by [deadline], and the reviewer agreed to review. This isn't a guarantee that the review will actually be submitted by that deadline, of course, but it is good enough to say that they are expecting to complete review by then.
Do reviews happen more in the holiday season generally? Assuming the reviewers have ample leisure time to study the paper?
In some cultures, everyone goes on holiday en masse during the Christmas season. It depends on your reviewer of course, but if they're on holiday, they presumably don't do peer review (which is more like work than holiday).
If your reviewer(s) are not from these cultures, then it's quite possible they continue to work as normal.
Does this indicate anything about the incoming decision?
No, aside from the fact that your manuscript is good/relevant enough that it isn't being desk rejected.