Montpellier's Stade de la Mosson will stage the first Heineken Cup semi-final while the second game will be at Twickenham or the Aviva Stadium.
The winners of Saturday's quarter-final between Clermont and Montpellier will take on the winners of Harlequins v Munster at the 32,000-capacity Stade de la Mosson in France's Languedoc region on Saturday, 27 April.
If Saracens defeat Ulster in their quarter-final, then they will meet the winners of the Toulon v Leicester Tigers at Twickenham Sunday, 28 April.
If Ulster win, the second semi-final will be played at Dublin's Aviva Stadium.
The Heineken Cup semi-finals are played at venues designated by ERC and the clubs who will secure home country advantage in the last four were determined by a draw which was conducted in January following the conclusion of the tournament pool stage.
Heineken Cup semi-finals
(Kick-offs local time)
Saturday, 27 April
Clermont or Montpellier v Harlequins or Munster
Stade de la Mosson
Kick-off: 18.00 (16.00 GMT)
Sunday, 28 April
Saracens or Ulster v Toulon or Leicester Tigers
Twickenham Stadium or Aviva Stadium
Heineken Cup final:
Saturday, 18 May