Seeing as there were some song sessions over the course of the weekend, it might well be the case that organisers simply struggled to find a venue willing to host such a session on the Saturday afternoon / early evening, perhaps.

Some landlords - even those who support folk festivals when they are on - want to keep some part of the day music free, so that their regulars, who may not want to listen to the music / singing, are not totally put off for the full day / weekend. Or maybe they didn't want to have to engage extra staff at periods when they're not usually busy, because they weren't convinced that there'd be any return.

Often festival organisers have tried to do many things that haven't come off, at the end of the day. Have to say, some of the comments about the lack of a singaround between x and y hours of the day seem a bit curmudeonly - in my humble opinion.