were to be held/would be held /were going to be held ,
在商店上面的一个屋子里,in a room above the store 这是地点状语.
where a party——（hold),一个PARTY 将要在这里举办,这是一个从句.
wokers were busily setting the table
工人们正忙着放桌子.
因为正“放桌子”,还没开始,所以是将来时,
整个句子是过去的,所以是过去将来时,
party被开,所以还要被动句.