For months with 31 days, all 31 days count.
This is also NOT a 60 day challenge to find caches 60 days in a row. If you recently started geocaching, on the 1st of a month with 30 days, this is certainly a great way to start the challenge requirements.
To complete the challenge, if your statistics are public, just state the months and I'll verify and email if your profile does not show you've met the requirements. If you prefer or if your stats are private, just post a stat similar to this one:
Feel free to find it anytime and post a write note log, but only log a find once the challenge has been met. Any find logs that do not meet the challenge may be deleted without notice.
Once you have met the challenge, edit the write note (or post a found-it) log with the above challenge requirements.