'Missa in Festo Sanctissimae Trinitatis' presents the reconstruction of a main religious service in one of the principal Hamburg churches at the beginning of the seventeenth century; it consists of works for ensemble and organ by the composer Hieronymus Praetorius, who was the organist at the Church of St. James in Hamburg from 1582 until his death in 1629. In between, the liturgical components of the Mass that were performed monophonically are heard as they have would have been in the Hanseatic city of Hamburg. The search for a suitable instrument led to the choice of the swallow's nest organ in St. Mary's Church in Lemgo; it may be regarded as a model example of the Northern German organ building tradition and has a mean-tone temperament ideal for the period of composition, characteristics, and stylistics of the selected repertoire.