I sit down at the end of the day and figure out what I am going to eat the next day. I try to get my calories between 1,200 and 1,800 depending on the cycling day. My problem is when on a low day I come back to make report on the things that I actually ate I have ended up with only 1,000 cal in a day, due to various reasons. This math usually occurs later in the evening. Should I go boost my calories with a healthy snack late in the evening or should I just try to do better the next day and maybe have a higher calorie day next?

I suggest you have the healthy snack.

Would it be possible for you to get a little notebook and keep track as you go during the day so that you'd realize it and be able to adjust earlier? I've had the same problem when I've waited til the end of the day to total up. When that happens I usually go by how I'm feeling - if I'm still hungry or think it will cause me to be in the morning, I'd have a snack. Otherwise, I don't worry about it.

If it's only once, I wouldn't worry. But if I do that for more than a day, I get sooooo hungry.

I'm going to say do a combined of the previous two posters posts. Track in a notebook, and eat a healthy snack if your calories are coming in that low.
Also try to plan!

Personally I wouldn't worry about it if you're not hungry or having any symptoms (and you're being honest with yourself about that), and it's not happening much.

Are you hungry? If so, eat. Otherwise, I'd let it go. I figure on my low days that in a day or two I'll be feeling extra hungry and "go over" on my calories.

Yeah I'd eat to my hunger for that day. I have some lower days and some higher days. I just wouldn't eat either too low or too high for more than one day if possible.

I completely agree with keeping tract through the day of what you are eating, even if it is on a small notepad. Take a section of the notepad in the back and write down some of your most common items and note their calorie counts for easy access.
When I find that I am low at the end of the day and I am not hungry, I don't eat. I just add the left over calories to the next day and try to catch up. Like most, some days I am over, others I am under....but I also tally up at the end of the week to see where I am. It let's me know if I need to adjust somewhere by reviewing my day to day journal and where the calories came from.