Purchasing humble monthly now (January 5th, 2018) will get you the February 2018 monthly bundle
The way that humble bundle markets their monthly bundle is somewhat unclear.
There are two sets of games in each monthly bundle: a publicized set that you get to "play now" as soon as you pay for the bundle, and a secret set that is revealed at the 5th of the next month. The secret set of games is revealed right after that month's bundle stops being sold.
The way the bundles are named corresponds with the date that the entire bundle is available, so the January 2018 bundle (which unlocked on January 5th, 2018), was on sale for most of December as well as from January 1st through January 4th. The bundle that you buy from January 5th until February 4th is the February bundle.
To sum up: If you want the Humble Monthly bundle for January 2018, you will need to go back in time and sign up in the past. If you want the humble monthly bundle for February 2018, you should sign up now. After signing up, you will get to play the publicized games, but the secret games in the February 2018 bundle will not be revealed until February 5th, 2018.