On Sunday I visited a special post office in Clausthal-Zellerfeld and was able to buy two special covers for my collection.
Saltwater Fishes (from set of three) (issued 04-08-2016)
with a special postmark about the 125th anniversary of the Biologische Anstalt Helgoland
Supplemental Stamp 8c (issued 03-12-2015)
175 years Stamps (issued 01-09-2015)
with a special postmark about the 150th anniversary of Heligoland's first stamps