Edinburgh, who travel to table-topping Leinster on Sunday, the Cardiff Blues and Munster, who clash in the Welsh capital on the same day, are all in with a chance of clinching the fourth and final place in the semi-finals.
Celtic Rugby Chairman John Hussey said: "I don't think we could have scripted the finish to the 2009/10 Magners League season any better than it has turned out.
"We already have a team from each of the three Celtic nations through to the play-offs and whichever team joins them will merely add to the calibre.
"While it was sad to see both Irish teams fall at the semi-final hurdle in the Heineken Cup last weekend, the Blues reached the final of the Amlin Challenge Cup by notching their fourth victory over a Guinness Premiership side this season at London Wasps.
"Munster are still the top-ranked team in the ERC Rankings and are the current holders of the Magners League trophy. They secured that prize via the conventional 18-match league route last season, but our 2010 champions will be crowned in a different way this time.
"The play-off concept has been wholeheartedly endorsed by the coaches, players, clubs and their fans and it means that every game has something at stake this weekend. Then we move on to the semi-finals and the Magners League grand final.
"Rugby fans love a big day out and the indications are that, whichever two of the six teams still in contention for a place in the Grand final survives through to May 29, it will be a game on a par with any other of rugby's other major finals this month."
The battle over the final weekend is not merely for the final place in the play-offs, but also for home advantage.
The top two teams in the regular Magners League season will enjoy a home draw in the semi-finals, with the highest-ranked survivor also having home advantage in the grand final.
"Not only do we have three teams battling it out for the fourth play-off place, but we also have four teams who could claim the two home ties - Leinster, the Ospreys, Glasgow Warriors and Munster," explained the Director of Celtic Rugby, David Jordan.
"It promises to be a nail-biting finish to our regular season with the final decisions on the make-up of the play-off matches running through to the final minutes of the season at around 7.45pm on Sunday night.
"Everyone has commented on the quality of the contests throughout the league this season and we have three fantastic weekends of action to look forward before the season ends."
MAGNERS LEAGUE - ROUND 18 FIXTURES:
Friday, May 7 -
Ospreys v Newport Gwent Dragons, Liberty Stadium, 7.05pm
Scarlets v Glasgow Warriors, Parc y Scarlets, 7.05pm
ULSTER v CONNACHT, Ravenhill, 7.30pm
Sunday, May 9 -
Cardiff Blues v MUNSTER, Cardiff City Stadium, 6.15pm
LEINSTER v Edinburgh, the RDS, 6.15pm
May 14-16 -
Magners League Play-Offs
ML 1 v ML 4
ML 2 v ML 3
Saturday, May 29 -
Magners League Grand Final