Both the set of Thursdays and the set of days are infinite integer sets. As the infinite sets share the same cardinality. That's assume they are infinite sets ofcourse, so I'm wrong if the universe ends. Although, if the universe ends my failures will be the least of our worries.
You can show the equality by counting them one to one.
You say the next day, I say the next week...
The one value will never outpace the other.