Christian ๐ฉ๐ช๐จ๐ณ๐น๐ผ๐ฌ๐ง๐ช๐บ Not that difficult... You first have to have money to gain interest. When you have had money in April, you will get the interest after that month. So May is indeed the first dat you'll receive interest.
If you had upgraded on say March 29th, you would only gained interest on that day and the past weekend. Payout of this very small interest sum would be yesterday or today.