They held Maccabi Tel-Aviv to a 1-1 draw in Budapest (Maccabi are from Israel but due to the situation over there, they are forced to play home European matches at a neutral venue) which took them through 2-1 on aggregate.
So on Wednesday 13th August, Chelsea travel to Slovakia for the first leg. Kick off will be at around 7.15pm UK time.
The return is a fortnight after that at Stamford Bridge...