"Yes, today is Thursday."
Translation:Sí, hoy es jueves.
This post will help: https://www.spanishdict.com/guide/using-the-definite-article-in-spanish. Use "el" when you're wanting to say something is happening on a day.
I was confused as well. So I turned to Google. I found this link https://www.spanishdict.com/answers/205749/use-of-definite-articles-with-days-of-the-week. The article is translated "on" so this is why it exists often before days of the week, but not here because the article exists after a form of "ser."