it charges you for the whole month.
if you signed up today, it'd charge you for the whole month of march, and not just for 2 days. because by becoming a patron you gain access to all the content available for that tier prior to signing up.
same thing goes for increases. if the creator increases their price near the end of the month, you're not just gonna pay that starting the next month, you'll be charged the extra money right away. so let's say you pay $5 a month for your tier and the creator increases that tier's value to $7 on the 30th, you'll be charged $2 right away and then on the 1st you'll get charged $7.
so if he waits until april 1st to post it, everyone will be charged for the whole month of april. which is likely to cause some people to stay, because they already paid for the month, so they might as well enjoy what he might post during april and cancel their subscription at the end of the month