The other semi-final between the second and third-placed sides will take place on Friday, May 11 at 7.35pm. The venue and teams are still to be confirmed over the next two rounds of the regular season.
Leinster Season Ticket holders can currently avail of a 25% discount up until Thursday, April 19 at 5pm via their Leinster Season Ticket account.
They will then have the option to buy tickets at full price up until Monday, April 23 at 5pm. Further ticket information will follow next week via www.leinsterrugby.ie.
Leinster have sewn up top spot with their 16 wins to date and will have home advantage in the play-offs. They will meet the fourth-placed team in Dublin and, if they win, will also enjoy home status in the final.
But the battle for the remaining three places is still up for grabs as the Ospreys, Munster, Ulster and Scarlets - four previous winners - and Glasgow Warriors, semi-finalists in 2010, battle for the right to keep their campaigns going beyond the regular season.
Even the sixth-placed Scarlets could possibly still end up with home advantage in the play-offs if they can win their final two fixtures and other results go their way.
The team that finishes second will host the team below them and, if Leinster lose their play-off, the winners of 2 v 3 encounter will host the final.
"The quality of the teams vying for top four finishes speaks for itself - four previous champions, three champions of Europe, two current Heineken Cup semi-finalists and two teams crammed with 2012 Grand Slam winners," said RaboDirect PRO12 Chairman Andy Irvine.
"Leinster have qualified for a home play-off, but the battle below them is going to be fascinating over the next few weeks.
"There is everything to play for, for those five other teams and we are guaranteed some thrilling twists and turns before the regular season ends.
"The RaboDirect PRO12 has enjoyed another exceptional season and has provided players like Alex Cuthbert, Justin Tipuric, Peter O'Mahony, Dave Kearney, Stuart Hogg, Dave Denton, Lee Jones and Matt Scott amongst others with the chance to graduate into international squads, and the Test stage.
"The final two games will match the quality, passion and intensity of anything that will be seen in England or France at the same stage of their domestic seasons and should provide the fans in the four nations that make up the RaboDirect PRO12 with a feast of rugby action."
Round 21 this weekend features two games between teams in the top six - Ulster v Leinster at Ravenhill on Friday night and the Scarlets v Munster at Parc y Scarlets on Saturday.
Ulster, currently fifth with 56 points, will have to rely on other results going their way if they do not win, especially as they travel to Thomond Park in round 22 for a repeat of their recent Heineken Cup quarter-final tie.
The Scarlets, in sixth on 55 points, end with two home fixtures, but need a minimum of eight points according to head coach Nigel Davies to stand any chance of making the top four.
They could do themselves, and the other teams around them, a favour by beating Munster and denying the reigning champions any points.
But Munster have won the last 12 games between the two teams in all competitions and are desperately trying to close the two-point gap on the second-placed Ospreys to try to earn a home play-off.
The Ospreys are all set for an emotional night at the Liberty Stadium on Friday as they mark the final home fixture for legendary winger Shane Williams when the Newport Gwent Dragons are the visitors.
If they can keep winning - they end with a tricky trip to Aironi - then they are guaranteed the second home play-off based either on points or wins.
Glasgow Warriors are also well in the mix and travel to Benetton Treviso this weekend and then end with a home game against Connacht. They currently lie in fourth place and could yet end up second with a home semi-final tie.