Is there a certification for Mariadb? Is there an official certification for Mariadb?
And if there is how can I find the next exam days?
I guess you can contact MariaDB to find out how/when/where you can take an exam. They state:

"This certification exam, proctored and administered by MariaDB"

on:

https://mariadb.com/resources/datasheets/certification-mariadb/

There is also preparation material to download there.