I think it's okay to call him and remind him. Give him the details so he has the proper information. He is their father. It would be wonderful if he attended for your daughters's sake. Just don't have any expectations from him and divert all of your attention to your daughters/children. Put all of your positive and loving focus on your children and their big day, for them. If you are there for them with your undivided attention fully engaged, your children will absorb all of that positive energy from you--the parent that is present! Sure, they may wonder about their daddy and miss him being there (if he doesn't show up). You can talk to them age-appropriately about daddy not being there and acknowledge their feelings of sadness or whatever they are feeling, if he doesn't show.
The most important thing is that you are there for them and they can count on you to share the important moments of their lives with a loving, engaged parent/adult.
When I reflect back on my own mothering (before my own recovery), I realized how much I focused on my alcoholic husband. I wanted to be the best mother I could be yet living with an active alcoholic, I realize how much my attention was on the active alcoholic (his actions, his inactions, his words, his drinking, his mistakes, etc.) I think I was inadvertently and unconsciously passing on my worries & concerns to my child (from the time she was in my womb). I used to worry about whether my husband would show up or not. I used to worry about whether my child would be hurt or disappointed. I tried to take on everything.
Even if we don't say anything, our children absorb our energy. I am an adult child of a sober alcoholic and married an A who is now sober. I realize how much I, as a daughter and wife of A's need to work on how I handle life situations so I can best cushion my child from any further harmful effects of alcoholism/addiction as well as do my best *not* to pass on co-dependent behaviors (and obsession with the parent/adult who isn't present).
Your husband is an adult. You can be courteous and considerate by reminding him and giving him the specific information of time, location, etc. Beyond that, it's up to him. Your daughters and you should enjoy the Open House to its fullest!