I have a product idea for a very specific niche (university students living on campus). When does the product validation take place? before creating a simple business plan and a website? or after having the website and social media running?
First of all, the fact that you are talking about market validation is great. I've seen so many startups invest time and money, only to find that no ones wants/needs their product, or that they only want a certain aspect of it, or they aren't willing to pay the requested price. So you're already one step ahead.
Answer: the sooner you validate your product, the better! That said, the version you are validating needs to be representative of the end product, or else the validation isn't reliable. So in your case, I would:
1. create a business model canvas (which is what all startups should start with before creating a business plan),
2. Setup a Wix or Wordpress website (this can be done for free / very low costs). On the website, include the price of the product, and enable people to order it (even if it doesn't exist!). I am happy to explain how this can be done even though you don't yet have the final product and by still being fair to the people who click the "buy/order" button.
3. Spend a small amount (say $100 - but depends on your budget) validating the idea.
This way, after only spending a very small amount, you will be able to know (if done right):
a. Do people like your product.
b. Do people want/need your product (not the same as 'a').
c. Are people willing to pay for your product?
d. How much are they willing to pay? (you can check this by having 2 landing pages for payment - each with a different price).
I'm happy to help you with this matter further, as this is a critical stage.
Best of luck!