Marathon Achievement in Microsoft Solitaire Collection
Come back each day of an entire month and complete at least one Daily Challenge.
How to unlock MarathonAs the achievement description says, you need to complete at least one daily challenge every day of a month on that day of the month. Keep in mind two important things: this must be a calendar month, not just 30 or 31 consecutive days; and you must do it from the first of the month to the last day of the month without skipping around.
Naturally you could just do this starting from the first of the month and log back in every day and do at least one daily challenge each day, but if you don't want to risk accidentally missing a day, it's much safer to simply change the date on your Windows 10 device to the 1st of an earlier month. Do at least one challenge that day, then change the date to the 2nd of the month. Repeat for every day of the month and the achievement will unlock after completing a daily challenge on the last day.
Note that it may be worth your while to combine this achievement with the "Strive for Perfection" achievement, which has you completing every daily challenge in an entire month. Since you have to do that anyway, might as well do them on all the actual day of the month to get this achievement too.