Eating 1000kcal extra in 1 day negates rest of week?

Your daily maintenance calories are NOT 2100...that is your BMR...

BMR is the amount you would need even if you were in a coma. As soon as you step out of bed in the morning, you require more than that for the day.

TDEE is the amount for maintenance (total daily energy expenditure).

This is your BMR (2100 for you) multiplied by an activity factor.
Even if you are sedentary it is X1.2
(all the way up to 1.9 for extremely active)

you burn 2500-3900 depending if it as a "lounge around all day" kind of day, or a killer workout and on-the-go ALL day kind of day...and anywhere in between....

so, where you THINK you negated those 6 days by one one "bad" day, you were actually bringing yourself up to the level you NEED to be eating at for the week for a healthy loss...

you should be aiming to NET 2100 a day (eat 2100 on rest days, and more on workout days depending on how hard you worked out that day).

One other think to keep in mind is that the numbers are all estimates. MFP is estimating your energy needs and expenditures. Unless you only eat prepackaged food and/or exactly measure everything you eat, you are estimating your portion sizes. Even then, your energy needs are still only estimates.

So what you need to do is evaluate your personal progress every three weeks or so. If your goal (losing or gaining) is not progressing fast enough (or maybe it's too fast!), then adjust your calories by no more than 500 per day, and see where you are 3 weeks later. I generally like to adjust by ~300 kCals. You can make the adjustment by any combination of diet or exercise (so long as you don't go below a required minimum number of calories). Even once you find a "number" that works for you, keep in mind that it will change over time. As you get closer to your goal (especially the last 5 lbs) the calorie deficit required may be much greater than it is in the beginning.

Also a lot of the current diet advice involves around having one cheat day a week, and it sounds like you used this. For some people a cheat day or meal is a great strategy, for others it doesn't work so well. You have to experiment - have two more weeks of "cheating" and see where you are - if no progress then either cut them out or have a bigger deficit during the rest of the week.

Finally, get a measuring tape; since I lift weights as well as run, there are weeks when I'm "cutting" where my weight goes up, but my waist measurement goes down. (For me this is the ultimate win!) Stick with it and you'll do great!

the Normal cheat day strategy worked for me early days, every Monday I'd be slightly heavier, then I would do it all again.
Now I've got to the point where I can pretty much binge and get rid of it in a few days, but that takes a lot of gym work, but then I continue with the same effort after, it becomes the new level. But thats after almost a year of doing this!