I would go ahead and put it on now. Sometimes after your frosting has crusted, it is hard to get them to stick on the cake. I have never had a problem with putting them on the day before!
I have put them on buttercream the day before and not had any problem the next day. The instructions say that you can put them on after the frosting crusts if you want, but you have to spray a fine mist of water on the frosting. I have not tried this but have heard that it works. Myself, I prefer to put it on before the frosting crusts. Hope this helps.
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