It’s well established fact (and a bit of a running joke) that the Doctor doesn’t always know what he’s doing, and that the TARDIS and his equipment often malfunctions. He was clearly oblivious to the harvest, and even if he had been, he could well have ended up at that particular point by accident.
For example: in The Girl Who Waited, he takes Amy and Rory to Appalapachia, without realising that when he takes them, there’s a plague that will kill them. Had he known about that, it would probably have been trivial to hop back a century or two, before the plague began. His awareness of current events was somewhat limited. Recall a conversation towards the end of the episode with Rory:
The Doctor: Rory…
Rory: This is your fault.
The Doctor: I’m so sorry, but Rory.
Rory: No! This is your fault! You should look in a history book once in awhile, see if there’s an outbreak of plague or not.
The Doctor: That is not how I travel.
Rory: Then I do not want to travel with you!
The Doctor isn’t just ignorant, he seems to revel in it. I think, for him, part of the fun is arriving somewhere and learning something new. Sure, he could have checked a history book and learnt about the harvesting of the forest, but that would take the “fun” out of it.
There are also lots of examples of his equipment malfunctioning. Whether it’s the fact that the sonic screwdriver doesn’t work on wood (particularly important in this episode), or the TARDIS landing in the wrong place/time (again, The Girl Who Waited, when he overshoots by forty years, provides a good example), his technology seems to fail regularly.
So let’s suppose that he knew about the harvest, and set up the present to avoid it (either arriving long before, or after the forest had been regrown); then there’s still a chance he’d end up there anyway. His ignorance and poor luck backfired on him badly.
Jumping out of universe, there’s also no reason to assume that the timelines move in parallel on either side of the box. There are at least two possibilities:
No matter when you open the box on the Earth, it always takes you back to the same point. You’d always open in the Forest at (say) 8:15pm on 28 January, 5012. This means that Cyril opening the box early prolonged their time in the Forest. Had they waited until the morning, they could have popped into the Forest for a couple of hours, seen what they wanted, and be gone long before the acid rain started.
The time streams could also move at different speed. An hour in the house could be a century in the Forest. So opening it in the morning might mean they arrive several hundred years later, or with just seconds passed. Either of those might be enough to avert coinciding with the harvest. (Indeed, it’s even plausible that the time streams could run in opposite directions).
Both of these possibilities have occurred reasonably regularly in the episodes (for variety, I can think of more examples from TGWW :-P), so they’re consistent with the logic of the DW universe. But they require more head scratching, so I find them a little less satisfying.
As for the surprise itself?
There are Christmas trees in snow. Christmas trees in snow are cool.
(W/o snark: he owes her a favour from saving his life, so why not try to make this Christmas, straight after her husband/their father’s death, a really good one. Whether that would actually help is up for discussion, but the Doctor can be blunt sometimes. I think he wanted to help, even if he went about it in a cack-handed way.)