I want to apply for a visa in order to attend trade fair on the 15th of April.

Christian, actually, your passport is not needed when booking the appointment as long as you have a photocopy of your passport data page at hand. You can submit the photocopy and book appointment on every Tuesday or Thursday afternoon between 15:00 to 17:00 p.m. at the embassy. After that, you need to wait for a few days to have an interview and your original passport and appointment slip are required when presenting at the interview. So if you can get your passport back before the interview, there won't be problems.

Thanks!