Okay, since your cycles are pretty regular and 28 days, the idea would be that you ovulate on day 14 of your cycle (with the first day of your period being day 1). That would be the 26th. This suggests guy B, but also suggests it could be guy A, because sperm can live in your body 5-6 days after having sex. Probably you had living sperm from both guys in your body on the day you ovulated. If the date of January 26th could have been stretched to January 28 for conception, it would rule out the first guy. But if your cycles are really pretty much like clockwork at 28 days, there's no evidence your ovulation would have been as late as January 28.
Is one guy a secret from the other? Because if not, the best thing to do to determine paternity would be to get each guy to pony up a third of the cost of a prenatal paternity test (Ravgen or the DDC) and to do swabs, and then you will know. Prenatal testing is quite expensive, but at least it's no longer dangerous to the baby (the guys do mouth swabs and you give blood from your arm). If you feel you simply can't tell one of them about the other, Ravgen also has a "discreet" test, in which you get the man's DNA sample by swabbing a drinking glass or his toothbrush. If you can't afford a prenatal test and would keep the baby no matter who is the dad, then just test when the baby is born. It costs only about a tenth of what prenatal testing costs. (Be sure to test with both men, so their tests will perform a cross-check on each other's, since one will come up yes and the other will come up no.)
One last way you could possibly get a clue about which guy is the dad, is if the baby is a girl or boy. You'll probably be told this at your 18th-week ultrasound. The reason this would help you decide who is the dad is that sperm carrying the chromosome to make the baby female swim slower and last longer, and sperm carrying the male chromosome swim faster and die sooner (it's said they die in about 3 days). If your baby is a girl, and if we assume the baby was conceived on the 26th, she would be likely to come from the first guy, and if your baby is a boy he would be likely to come from the second guy. This all depends, since we don't really know for certain you did ovulate on the 26th, and who knows, there might be some fast-swimming female-carrying sperm out there in nature, or some slower-swimming male ones. But that's the theory. You can see why a DNA test is lots better.
When you say "the weekends before that," do you mean January 21 and 22?
Also, how often do your periods come, are they super regular and arrive on the dot every 28 days, or do they vary?