Ces fêtes ________ incroyables.
It seems to me both Imparfait and Passé Composé could be used here, for different meanings.
"Ces fêtes étaient incroyables." Those parties [which happened regularly, a long time ago] were amazing.
"Ces fêtes ont été incroyables." Those parties [which were not long ago, and which happened only once (e.g. in a specific summer)] were amazing.