End of Feb if the past 3 years have anything to say about it.
2017 - drop Jan 4th, back up Feb 17th
2016 - drop Jan 10th, back up Feb 20th
2015 - drop Jan 12th, back up Mar 3rd
I have also observed it last year when I thought bitcoin went down on January after a good pump on December (2016) and then it started to rise in the mid-February up to how it was on December (2017). Nothing new. Just another opportunity to fill our bags.