Welcome to the Cake Decorators Q&A

Hi everyone,
I have 3 cakes to make for Saturday and I plan on staggering the decorating making the easy one first to get it out of the and taking my time with the 2 harder ones. my problem or worry is that i use swiss meringue buttercream for my cakes. I had some left over egg white so i make my swiss meringue with egg white. now i am worried about how long the buttercream will last and if i am wrong in using egg white. if i cover the cakes now will I end up causing hard when eaten on Saturday?
I hope I am making sense and that someone can advise me about egg white in swiss meringue.
Many thanks everyone and have a good evening!
Davina
In Switzerland

Me again, I forgot to add that the cake will be filled with swiss meringue but covered with chocolate ganache and then fondant. I really don’t know if this makes a difference if I were to leave the fondant covered cakes out in a cool place.
Thanks again.
Davina