Polygyny permission has been granted. Should I tell my first wife the date of the akad and the ceremony?

Yes, tell her early; finding out from someone else will feel like being betrayed all over again. She has already been through the court process, so there are no more secrets to keep. Choose a calm time and tell her the date, place and your plans for the days after, including the rota arrangements and who will be with the children that day. Do not force her to attend, and do not forbid her if she wants to. Let her choose what best protects her heart. Avoid showing off the ceremony in a big way on social media where your wife and children can see it. See walimah.