As soon as you sign up for Ethnio, you will pay for your first month. After a month goes by, regardless of the day, you will then be billed again for the next month. So if you sign up for an Ethnio account on the 1st of April, you'll immediately pay for the month of April, and then be billed again on the 1st of May.
Forget your billing date?
Once logged in to your account, go to your Account >> Plan & Usage and you'll see the billing date
Example of what to look for: