The 23nd RAGtime workshop will be held in Opava (6-10 September 2021) in hybrid form. Please, follow this website for further informations. All participants on site must present a proof of vaccination or negative test to the organisers upon arrival and registration to the conference! The series of the annual RAGtime workshops has been held at the Institute by the Relativistic Astrophysics Group (RAG) since 1999. Its scientific scope is devoted to current problems of relativistic astrophysics mainly focused on the physics of black holes and neutron stars. The workshop is attended by multiple experts in the field and allows for many valuable discussions and interactions between the participants. The scientific focus of the workshop will traditionally be devoted mainly to problems of relativistic physics of black holes and neutron or quark stars. Large attention will be given to confronting theoretical models with up-to-date observations available through both electromagnetic and gravitational wave window to the Universe. Special attention will be devoted to the modelling of astrophysical effects in the combined strong gravity and relevant external magnetic fields. Current issues in accretion theory will be explored. We will furthermore attempt to address problems related to cosmology, mathematical aspects of the theory of relativity, and alternative theories of gravity. In addition, new methods of generating complex general relativistic solutions containing a black hole will be exposed. We will also focus on the national and international collaboration regarding the present and future cosmic X-ray missions.