The welcome reception on September 3rd will take place at the Žofín garden, a covered open-air restaurant on Slovanský Island on the Vltava river with an impressive panoramic views of the Prague Castle, the National Theatre and of other historic monuments of Prague. The reception opens at 6PM and is expected to close at 9PM. You can stay even after 9PM, but the drinks (at discounted rates) will already be at your own expenses.
The address is Slovanský ostrov 226 (see the map below). The island is located very close to the tram station Národní divadlo (trams no. 6, 9, 17, 18, 22, 24). You can check the connections here.