They welcome Leinster to Ravenhill on Friday night, a victory for the visitors would close the gap between themselves and the leaders to single figures. But a loss could put their own top four place into jeopardy.
The other all-Irish fixture is on Saturday when fourth-placed Munster travel to the Sportsground to play Connacht, who are looking for a clean sweep over the three other provinces in Galway this season.
Munster have been in the top five of the table all season, but any slip-up in Galway could let Glasgow Warriors or the Ospreys take over their coveted play-off spot.
There is only one Welsh derby before Christmas as the round 11 game between the Newport Gwent Dragons and Ospreys has been held back until New Year's Eve, so the focus on Friday night will be the second-placed Scarlets' trip to Cardiff Arms Park.
The Scarlets need to be aware of the teams both above and below them in the table. They will be anxious to close the gap on the leaders and must be aware that any slip-up could see them lose the home play-off berth they currently hold.
The remaining game on Friday night is the first leg of the Glasgow Warriors v Edinburgh double header.
Honours were even in the first game last Christmas, with the Warriors going on to take the second leg, and with the home team still very much in the running for a top four spot, there is guaranteed to be a capacity crowd at Scotstoun Stadium for this encounter.
The only remaining game before Christmas will be in Italy on Saturday - it is the first ever local derby between Zebre and Benetton Treviso.
The hosts are still looking for their inaugural win in the competition, but Treviso, who were minutes away from pulling off the shock of the last round of European games against Leicester Tigers, will be a hard team to beat.
RABODIRECT PRO12 - ROUND 11 FIXTURES:
Friday, December 21 -
ULSTER (1st) v LEINSTER (3rd), Ravenhill, 7.05pm (live BBC 2 Northern Ireland/RTÉ Two)
Cardiff Blues (9th) v Scarlets (2nd), Cardiff Arms Park, 7.05pm (live BBC 2 Wales)
Glasgow Warriors (5th) v Edinburgh (7th), Scotstoun Stadium, 7.35pm (live BBC ALBA)
Saturday, December 22 -
CONNACHT (10th) v MUNSTER (4th), the Sportsground, 5.15pm (live TG4)
Zebre (12th) v Benetton Treviso (8th), Stadio XXV Aprile, 3pm (live SportItalia)
* The Newport Gwent Dragons v Ospreys match will be played on Monday, December 31