Does Ribblr contact you or do you contact them?
You opt in your shop for Flash Thursdays if you want to be in them It should be somewhere in your shop manager : )
Hope this helps!
Edit: Actually it’s random too! See @saar ’s post below for more info.
It’s randomly picked, but you are likely to get picked if you have quite a lot of patterns.
Here’s how you opt-in:
I was wondering this same thing the other day!