This app allows you to calculate the next possible date for your next blood donation. You have to set your gender to get the correct date. Additionally, you can manage all your blood donations. Based on this information, the app calculates the next date when you are allowed to give a blood donation, based on the information on https://www.blutspendedienst-west.de/blutspende/blutspendetermine/rechner_abfrage.php.
You can also get the dates for the next blood donations from the DRK.