Why do Flipkart and Amazon keep their Big Sale festivals on the same days?

Imagine you are a lemonade stand owner on a long beach.
You notice that customers come only from one side (let’s say the right side). Where would you place your shop?
On the right side.
Now, a friend of yours sees that you are doing pretty well. Where will your friend open her lemonade shop?
Exactly right next you, because she doesn’t want to lose customers who will come to you first.
What if you had customers coming in from both sides? Where would you place your stand?
Right in the center.
Now think about your real life experiences. Why do you see McDonald’s so close to Burger King? Or car stores right next to each other? Or furniture shops all on the same strip?
This is nothing but central place theory or the reason why malls or cities exist.
In space, you will be there where your competition is. You will do the same in time. If your competition is attracting similar customers at a particular point in time, you will ensure that you are there as well. This is especially true if customers believe what you and your competition are selling are very similar.
Later/farther than your competition means losing customers.
Amazon & Flipkart need to have their Sales on the same days, or they lose.