I'll try to explain the MIDN pay system. wilson73 did a pretty good job but I'll try to elaborate. It's complicated, so bear with me.
MIDN pay is a strange beast. Technically, you always make the same amount of money all four years (i.e., for tax purposes). According to my LES, this is $974 a month. It's what you see that changes.
A number of items get deducted from this pay, and it all gets broken down on the LES:
-Federal Taxes
-Social Security
-"Personal Deductions"--this one always causes some consternation. Personal deductions covers stuff like haircuts, the laundry service, and the tailor shop. As someone who has never gotten a haircut on the yard (don't trust it), barely uses the school laundry service (do my own), and can count the number of times I've used the tailor shop on one hand, I'd rather have my $119 a month back.
-State taxes
-Alumni Association
-NAAA
-MID Card charges (textbooks, uniform store, etc.)
-Class Fund
-And, of course, the ACE loan.
The ACE loan for my class was around $7500. I have a couple friends who either applied scholarship money or had enough saved up from working in high school to pay it off either partially or in full. According to my 2/C year LESs, the ACE loan deduction was like $200 a month. I can't access my older LESs and so can't speak to before then. If you don't pay off your ACE loan separately, it gets paid off by like October or November 1/C year.
Held pay: During 4/C through 2/C year, money is also "held" back from your monthly paycheck in case you need it for the uniform store, textbooks, or for certain items at the Mid Store. The amount this is decreases and you become less eligible to buy certain things with it at the Mid Store (uniforms/textbooks are always good to go). Every once in a while, you get this back in chunks (held pay). Held pay also includes COMRATS (commuted rations), which you get for leave periods and is basically the money that the Navy saved by not having to feed you on the Yard. Held pay is usually distributed in January and May, if I'm remembering correctly. The theory is that in May you're about to get released to the world on summer training, which can be a big money sink. Also, you don't get paid the next class up paycheck until the plebe class gets its first paycheck, so August.
General Pay:The trick is, during 4/C through 2/C year, you ONLY get the $100, $200, or $300 a month. I'm honestly not sure if you get the ACE loan money back before 1/C year if you pay it off, if yes then that would be an extra ~$200 a month.
During 1/C year, your pay is your entitled base pay (the $974) minus the deductions, so there's not really any held pay and it varies from month to month. So, for example, if you decide to buy textbooks 1/C year using your Mid card, it comes more-or-less directly out of your paycheck rather than the nebulous "held pay" bucket that you get at random times throughout the year.
Clear as mud? As a mid, you can also go down to the disbursing office and have them explain your paycheck to you.