USUALLY if you miss a day or start after the 1st, you have to wait for the upcoming month to start the counter. However, i did all the challenges for this month, and my progress is now showing me 21 of 31, so be quick and complete them all before an update fixes this.
Like the achievement says, there is only 1 needed a day and YOU HAVE TO (USUALLY) log in and do it.
If you are unsure just log in again.
Note: After finishing all the challenges, it showed me actually 20 of 31, but then i completed a challenge from today (21st) and i had 21 days of 31 again. So double-check and do a challenge AGAIN if the counter is wrong.
Note2: Look for the comments. Multiple gamers report, that this is actually glitched.
EDIT: This achievement unlocked at the 26th day of the month - which means a whopping 5 days earlier than it usually should unlock.
Like i said, use this to your advantage before an update fixes this.