In Rome will you only find gnocchi on a menu on Thursdays?
It used to be that restaurants only had gnocchi on Thursdays. This is changing though and you often find them on menus other days of the week. That said, if you want to be sure, you can always either call ahead, or - as you suggest - change your day.
But...just because it's thursday, doesn't necessarily mean you will find gnocchi. So best to ask anyway.