My potato salad is always watery 1 hour after making it. How can I make it dry for at least 2 days?

I have never in 50 years had that happen until yesterday. I decided to add some sour cream to the mayo for the first time and, voila, 3 hours later when serving, tons of liquid in the bottom of the serving bowl. It had to be the sour cream. I will never make that mistake again. I see recipes that do the combo, but that was the result I got.

Potatoes do not exude water in any significant amounts. Mayonnaise breaks down into oil and some solids.

I salt my celery before using it in salads. If you salt and mix it, let it sit, it will exude water. Then I rinse it and squeeze it in a dishtowel or strong paper towel. Since I have started doing this, I have never had watery salads.

If you put the salted celery in a strainer and bowl, you will see how much water it produces. Likely the same amount as you are seeing puddle in your salad. For my various salads, celery, cabbage, and cucumber are the major producers of water

Salting vegetables creates an exchange. Water for salt. The vegetables loose some water, and gain some salt. The salt they gain equalizes them with the saltiness of the dressing and prevents the dressing from pulling out more water.

Make sure your potatoes are completely cooled before dressing. Hot potatoes are still giving off steam, and if they're dressed, the steam goes into the dressing instead of the air.

Celery gives off a lot of water. Either eliminate it, or chop, salt & drain it well in advance of adding it to the dressing. You could use celery seed in place of celery - although not exactly the same, there is a similar subtle underlying bitter bite to both.

And what are you putting into it besides potatoes?

How are you preparing your potatoes? Are you peeling, cutting them up and boiling them? Try steaming them with their skins instead, then peel them while still warm. Generally waxy potatoes are better for potato salad, like red or Yukon Gold.