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It sounds like he must have been over-tired. When someone starts sniping at you, and there doesn't seem to be any real reason, and you just want it to stop before it gets out of control, just say: " You may be right".
This response doesn't actually say you agree with him, but it will take the steam out of many arguments. Fights escalate when each one is trying to defend themselves---if one person stops defending, the fight usually stops.
You and your husband may want to read the book "Non-violent Communication" for ways to express yourself that get your point across without putting the other in a defensive place where they have to defend themselves. If you could both learn this way of communicating, weekends would be a lot easier for you.
In the meantime, now you have some valuable information...when he works more than 2 days he gets tired and cranky. Now that you can predict these moods, you can become proactive and make sure that he gets enough "down time" after work on those days, maybe even plan his favorite meals for the weeks he works extra. You want him to get back in the habit of working more whenever possible, and men respond much better to rewards when you want them to keep doing something. The weekends after he has worked extra, make sure not to leave him alone with the baby for long stretches...you will both need some time to yourselves.