I don't know what it is, but all hell seems to break lose around here when 5:30pm hits. DD1 is still cranky from her nap (who is cranky after a 3 hr nap, anyway??) and starts making demands for cookies and junk that I won't give her, and then she starts screaming. DD2 is, well, a baby, and that's when she usually starts with her early evening fussy time. And all of this is happening when I'm trying to think about getting dinner started. Instead, I end up parking DD1 in front of the TV with a snack, while I try to settle the baby down for a little rest, and we don't end up eating dinner until 7:30.
During all this, DH is either wrapping up work, studying, or at class. I just feel so horribly stressed at this time of the day, every day, and I really think I need to talk to him about rearranging his schedule so I have another set of hands. I actually started crying when both girls were crying earlier, and I felt like such an idiot.
That's all, just needed to vent a little bit. Today is his long day where he's gone for 12 hours, and we didn't make it out of the house today, which I'm sure is compounding my stress! I can't wait until the baby gets on a more definite schedule so I don't feel so haphazard hour to hour.

Re: 5:30 is an ugly time of day
Would it help if you made or just prepared dinner during afternoon nap? That's what I do when DH is working until 6. I either prepare a casserole then throw it in an hour before or chop everything and throw it into pans before he gets home.
Things will get easier, I promise. I cried many tears when Liam was 2 and Jack was a baby.
That's just a crummy time of day in general. It's like everything HAS to be done between 5:00 and 7:00, and it's the WORST time for kids. I second the recommendation to do dinner prep earlier in the day.
Also, my salvation most days is that DH cooks dinner. We had that discussion a long time ago when I got home later than he did when I was a student and pregnant. He saw how stressed I was all the time and was understanding.
Hang in there! Hopefully you can work something out!
I did talk to DH last night, and he also suggested that I work on dinner during DD1's nap and we just eat early dinners going forward. That will be a big change for us, we've always eaten late since he hates early dinners. But we're going to give it a try and see if it takes off some of the pressure.
I used to always do my meal prep during her nap, but I've gotten away from that since the baby was born. She is usually awake during DD1's nap, and for the half hour that I can get them to sleep concurrently, it's tough to get motivated to get myself into the kitchen. I'm going to make a better effort, though, and I think I'm going to try more meals in the crock pot, since it gives greater flexibility on our dinner time.
I know things will ease up once the baby gets on a more reliable schedule, I just need to hang tight for now. Thanks for all of your thoughts!