You need to complete a minimum of one daily challenge each day for a month, the level of the challenge completed does not affect the achievement, for example, completing an easy challenge each day will work.
As long as you complete a month's worth of challenges in date order sequence as per the achievement description it will unlock. You don't have to start on the first day of the month. This was true for myself and others on the recently obtained list. Completing the challenges in date order sequence is important, don't complete them in reverse order or out of sequence if you want it to unlock. I say this because I think completing challenges out of date sequence probably resets the day counter for the achievement.
For example you can start mid February and unlock the achievement mid March. I say this because I unlocked mine on the 12th, others unlocked it on the 27th and 1st etc. I think in my case the PC identified a sequence of daily challenges in place starting mid January. When I repeated a sequence of daily challenges starting on the 1st February the console added these on to the January sequence. So when I reached the 12th I had a long enough sequence of challenges to unlock it. I am not sure exactly how many days are counted as a month, I guess somewhere between 28 and 31.
Also the system clock does not have to be set to the present day to unlock, mine unlocked with it set on the 12th February, when the actual date was the 28th. You can also set the system clock to an earlier date (I think two months is the maximum) if you want the achievement quickly. Again just do them in date order sequence for the reason I mentioned above, repeating previously obtained daily challenges also works. Once I completed a daily challenge and set the calendar to the next day I logged out of Windows. I then logged back into Windows and completed the next challenge. I did this as a precaution to ensure that the Solitaire server recognised the new date for the next daily challenge.