The challenge is approved, when you have find a geocache (any type) on a specific day every month according to the year.
Only the years 2001 till 2028 are possible.
Example for 2019: Approved when you have find a geocache on 19th of January, 19th of February,…, and 19th of December.
Same for 2015: Find a cache on 15th of January, 15th of February,…, and 15th of December.
Only one fully completed year is needed.