Saints face a five-day turnaround in Heineken Cup Pool 1 in January after the fixtures were announced on Wednesday afternoon.
Jim Mallinder’s men head to Ospreys on Sunday, January 12 (kick-off 12.45pm) before returning to Franklin’s Gardens to take on Castres on Friday, January 17 (kick-off 8pm).
Leinster face a similarly tough schedule as they go to Castres on January 12 and host Ospreys five days later.
Those games will decide the destination of the pool, with Saints currently four points adrift of summit side Leinster.
Northampton produced a stunning performance to claim an 18-9 win in Dublin last weekend to keep their hopes alive.
And they will now be targeting back-to-back victories, while hoping their Irish rivals slip up as they bid to make the quarter-finals for the first time since the 2010/11 season.