...the mall was open/ opened.

If you mean it was the first day that the mall was available for customers to visit and make purchases, you need 'open', which is an adjective, not a verb. A mall, or a shop or store is either open or closed:

The mall was very crowded with shoppers. It was the first day the mall was open.

If you mean it was the day that the mall was officially inaugurated then that can only happen once, so the use of 'first' is incorrect, and you would use the 'opened' past participle (a verb):

The mall was very crowded with shoppers. It was the day the mall was opened.

Thanks, Tes yeux noirs.