We plan to stop in Graz on our way to Vienna and noticed that we will arrive on Whit Sunday and I'm wondering if the fact that Monday is a holiday will have places closed that we might want to visit. Any feedback on what to day there would also be welcome. Thanks in advance

Have you googled the web sites of the places you want to visit? That should provide opening days.

Stores will be closed on Whitsunday (like on any other sunday) and Whitmonday..
Otherwise, it will be business as usual for museums, attractions etc. In fact, there may be a bit more visitors than on a usual working day, just like on a sunday.

Maybe there will be the odd exception from the rule, but you can check what you can do and visit here:
https://www.graztourismus.at/en

Thanks for the response and the suggested website

Love Graz. Eggenberg castle is worth going to. Did some walking tours of the city even though they were only in German - we did have a German friend who could translate with us.
Enjoy the Stryian culture