We did the ABC trek in late December 2017 (Christmas time). Although tecnically winter (December, January and February are regarded the winter months in Nepal) the weather was very warm and sunny, which is quite unsual for that time of year.
Generally in winter you should expect snow and spells of bad weather and pack accordingly. We did have gear for bad weather with us, even if we didn't use it. Having said that, although those months are colder there is usually less cloud coverage (blue skies) and it's also significantly less busy, which is very welcome as this is a very popular track.
Should you encounter bad weather during the hike, the areas just before and after Deurali is where you need to be careful as in case of avalanches there is nowhere to hide.
I always wondered what people meant by "be careful" if you're fully exposed in the worst case scenario anyway, but this is my take on it: Ask locals. If the risk is too high the track will be closed anyway from that point onwards. Otherwise look for snow, use the alternate route if necessary (on the other side of the gorge, right hand side if you are looking up) and don't dilly dally. You should be especially vigilant if there was heavy snowfall recently (winter) and during the first weeks of spring (due to melting of the winter snow). Those precautions should keep you reasonably safe, albeit not 100%. But hey, that applies to life in general.
We did the ABC trek directly from Poon Hill, which is why we are starting at day 2 here. You can have a look at the Poon Hill part of the hike here. The Poon Hill trek is optional and will add 1 or 2 days to your trek in total.
Day 2 - Ghorepani to Tadapani
8 hours - 7 km
This is where we get into some of our longest days of hiking, not intentionally but more forcefully by me towards B. Today we started at Ghorepani and headed towards Tadapani (2540M). Along the trek we saw many Mules, some which would not move and you had to wait for them to decide to pass. This part of the route you have many spectacular views and you end up at one of the most beautiful places on the trek to stay. Hanging out that night with hot chocolate and various people who you had met on the trek was a different way to spend Christmas. (I have to say the meals on the trek were fantastic, so delicious and exactly what you would want at the end of a long day).
Day 3 - Tadapani to Lower Sinuwa
10 hours - 9 , 5 km
Waking up in Tadapani and looking out on your deck to the sunrise makes you feel like you are on top of the world. You have had no cellphone or contact with the outside world for two days now and it just frees your mind. From Tadapani (2540m) we set out early at 9am and spent the rest of the day hiking till 6 pm at night, going through Chomrong (2170m) and then via the longest stone staircase steps ever to ending up at lower Sinuwa (2340m I think a bit lower).
This hike was all up and down with a lot of stairs, but was so worth it. About 3 hours into the walk we were lucky enough when … two red pandas went running out in front of us across the path and straight back into the bush. The most unbelievable sight ever. But we were also completely smashed by the end of that day and Sinuwa provided us with our worst accommodation we had, had on the whole hike.
Day 4 - Lower Sinuwa to M.B.C
11 hours - 14 km
By now it was Day 4 on our hike, we were tired but had a goal to reach Deurali (3100m). So we woke up early and started walking, stopping around 9 am for breakfast at Bamboo (2335m). Then we marched on to Doban (2505m) where we stopped to a packed camp site and decided to eat lunch before carrying on. By the time we reached Deurali it was 4pm and the camp was almost fully booked out by tours (watch out for this as people with guides book ahead, we had no guide so risked turing up to no accommodation).
Here we meet four guys who convinced us by saying MBC is only 2 hours away to hike the remained to make it to base camp. This was probably one of the silliest things we did on the hike. We reached MBC (3700m) just before nightfall and it was getting cold and dark. (The weather the whole hike was perfect and sunny, but night times were still super cold. We got very lucky with our hike as days later snow was predicted). We fell down on the couches and ordered a meal while watching the sunset over fishtail mountain, a surreal experience. About an hour later the guys turned up cold and hungry. We didn't get altitude sickness as we had taken tablets the day before, but looking at our video footage our faces were swallow like balloons.