Weekend Weight Gain

Other possibility: are you perhaps doing some sort of exercise that you aren't doing during the week? Causing muscle soreness?

Yes: Water weight changes, possibly some contribution from waste still in the digestive tract.

Rules of thumb:

Multi-pound changes over a day or few, without a dramatic change in eating or activity: Usually water weight fluctuations, with maybe a little assist from food waste.

Bodies can be up to 60%+ water: That's a lot. Different amounts of waste-to-be in your digestive tract has weight, and that weight also varies. An apple in your stomach weighs the same amount as it did in your hand, until it finishes its digestive transit. A 150-gram (1.8 ounce) apple only has about enough calories to create 0.036 ounces or 10 grams of body fat (and that only in a surplus), but you'll weigh 150 grams more right after you eat it.

Fat loss (with a consistent calorie deficit) is a few ounces or a few dozen grams per day, and plays peek-a-boo on the scale with the bigger, quicker water weight shifts.

Even fast fat loss is slow: 2 pounds a week is about 4.6 ounces per day (equivalent weight to a half-cup of water). A kilogram a week is about 143 grams, so weight equivalent to 143 milliliters of water. It take roughly 3500 calories of cumulative deficit to lose a pound of fat, roughly 7700 to lose a kilogram.

Fat loss will show up in weight trends over several weeks or longer. Loosely, that's saying that one's average weight over a week or so will be lower in a few weeks, if fat loss is happening at a pound or two (half to one kg) per week.

Really good muscle gain (under ideal circumstances) would be up to a couple of pounds (a kilogram) per month. Ideal circumstances include a good progressive weight training program faithfully performed, favorable genetics, relative youth, male hormones, good overall nutrition (especially but not exclusively ample protein), relative newness to strength training, and a calorie surplus. It's not that a person can't add muscle in less ideal circumstances, but it would be slower.

Sometimes people mistake strength increase for muscle gain, but strength gain precedes muscle gain. (Early strength gain is from better recruiting and using existing muscle fibers, a.k.a. neuromuscular adaptation. Muscles may look more defined before muscle mass is gained because of the water retention in the muscles for repair, plus maybe a little initial fat or water-retention decrease from reduced calorie intake.)

This is a good read, especially the article linked at the start of the thread:

https://community.myfitnesspal.com/en/discussion/10683010/the-weird-and-highly-annoying-world-of-scale-fluctuations/p1

If the scale weight gain can't be fat, it's not worth worrying about.

ETA: Let's hope I did the arithmetic in this post correctly, because sometimes I don't. If I didn't, please correct me.

I also experience weight upticks after eating out.

I always figured this is because the fundamental goal of a restaurant is not necessarily to serve you nourishing and healthful food, but rather food that gives you pleasure and satisfaction so you keep coming back for more.