No need to wonder. Fortunately, the
Gun Violence Archive allows you to export all mass shooting data for the year.
A quick Pivot will show that (surprise!)
Texas has had the most mass shootings on Sundays so far this year.
This list is just the top 10.
Edit: that didn't actually answer your question though. You can pull the data down yourself, but in almost every state Sunday is the worst (likely because they happen after midnight on a Saturday).
Edit 2: Awe Reddit, you're so cute. Downvoting factual data that's been posted without bias. Stay classy guys.