Longest I've worked without rest + why?

Over 24 hours. Why?

In our old system, the database had fields, for example, a customer like Total97, Total98, etc. to store values by year (or some date-specific value).

Every January 1, we had to add fields to accommodate the upcoming year and make the appropriate code changes to handle the new fields.

One year the UPS shipping rates changed and users didn't want to 'lose' the old rates, so they wanted new fields added (Rate98, Rate99, etc) so they could compare old vs. new. That required a complete re-write of most of the underlying applications because users wanted to see the difference on any/all applications that displayed a shipping rate. I'll throw in asking 'why?' was often answered with "because we pay you to do what we say". Luckily, we had already gotten to work on a lot of this before January 1st, so we were, for the most part, ready.

January 1st rolls around (we had to be in the office at 3:00AM), work thru changes, spend some time testing, and be done before noon. That didn't happen. The accounting system was a system that wasn't in (and had never been) in scope, and when we flipped the switch, one of the accountants comes into the office:
E: "Guys? None of our Excel spreadsheets are working. They are critical to integration with the accounting software"
Us: "What? Why would you be using Excel to integrate with the software instead of their portal?"
E: "We could never figure it out, so we had a consultant write VBA scripts to do the work."
Us: "OK, a lot of fields changed, but shouldn't be a big deal. How many spreadsheets are we talking about?"
E: "Hundreds. We have a separate spreadsheet for every integration point. The consulting company said it scalable, whatever that means."
Us: "What?! Why we just know hearing about this!?"
E: "Don't worry, the consultant said making changes would be easy, let me show you, just open the spreadsheet..click here..<click><click><click>...ignore that error, it always happens...click that <click><click><click>.."
Us: "Oh good lord, this is going to take hours"
E: "Ha! Probably. All this computer stuff is your job and I've got a family to get to. Later"
Us: "Hey 'VP of IS', can we go home and fix these spreadsheets as-needed this week?"
VP-IS: "Let me check with 'VP-FS'"
<few minutes later>
VP-IS: "No, he said Excel is critical to running their department. We stay until Excel is fixed."
Us: "No, no...its these spreadsheets. I doubt FS needs all of them tomorrow morning."
VP-IS: "That's what I said. Spreadsheets, Excel, same thing. I'll order the pizza. Who likes pepperoni!?"

At least he didn't cheap out on the pizza (only 4 of us and he ordered 6 large, extra pepperoni from one of the best pizza places in town)

One problem after another and we didn't get done until almost 6:00AM. Then...

VP-IS: "Great job guys. I've scheduled a meeting at 8:00AM to review what we did so we can document the process for next year. You've got a couple of hours. Feel free to get some breakfast and come back, or eat the left over pizza in the breakroom fridge. There is a lot left"
Us: "Um...sorry...we're going home."
VP-IS: "WHAT!!...OK...fine. I'll schedule the meeting for 12"
Us: "No...we're going home. We'll see you tomorrow."