On August 17th 2017 the first electromagnetic counterpart of a gravitational wave (GW) event originated by the coalescence of a double neutron star system (GW 170817, Abbott et al. 2017) was finally observed. A world-wide extensive observing campaign was carried out to follow-up and study this source. In this talk I will describe our unique spectroscopic dataset acquired with the VLT which allowed to characterize and identify the optical counterpart of GW 170817 as the first compelling example of a "kilonova", a transient source powered by radioactive decay of heavy elements resulting from the r-process nucleosynthesis of ejected neutron star matter. All the activities I will describe are expected to provide means and opportunities to all the European astronomical communities to have a leading role in the GW astronomy and Time Domain Astronomy.