There are three games on Friday night, including two Scottish-Welsh affairs with Edinburgh facing the Newport Gwent Dragons and Glasgow Warriors hosting the Scarlets.
Whilst mathematically neither Welsh region is yet out of the running for the play-offs, both teams will also have an eye on securing the third Welsh Heineken Cup place for next season.
The Dragons have a game in hand over the Scarlets, whose recent poor form has seen them slip to sixth in the table having spent 12 consecutive weeks in the play-off zone.
Also on Friday, Ulster, whose loss to Leinster last weekend dropped them out of a home semi-final spot, face another of their Irish rivals, Connacht.
With only three points separating the second to fourth-placed teams in the table, and with the Blues having a game in hand over them, any loss of points at this stage could be crucial for Ulster's play-off ambitions.
The remaining two teams who are in play-off contention, Leinster and the Ospreys, are both in action on Saturday.
Leinster, who did not climb into the top four until round 11, travel to Italy to face Aironi Rugby. Joe Schmidt's men will have to be careful they do not slip up to the Italian outfit who recorded their first Magners League victory over Connacht last month.
Reigning champions the Ospreys, on the other hand, are hosting league leaders Munster at the Liberty Stadium, a ground where the hosts have not lost to an Irish province since September 2009.
MAGNERS LEAGUE - ROUND 21 FIXTURES:
Thursday, April 21 -
Cardiff Blues v Benetton Treviso, Cardiff City Stadium, 7.05pm (live BBC 2 Wales)
Friday, April 22 -
ULSTER v CONNACHT, Ravenhill, 7.05pm (live BBC 2 Northern Ireland/TG4)
Edinburgh v Newport Gwent Dragons, Murrayfield, 7.30pm
Glasgow Warriors v Scarlets, Firhill Stadium, 8.05pm (BBC ALBA/BBC 2 Wales)
Saturday, April 23 -
Aironi Rugby v LEINSTER, Stadio Zaffanella, 2.30pm local time/1.30pm Irish time
Ospreys v MUNSTER, Liberty Stadium, 6.30pm (live RTE 2/S4C)