"The party yesterday was fun."
Because the i-adjective 楽しかった（たのしかった）is what indicates past tense here. The ~かった ending means "it was ~". Since this is the basic/informal form, です is added for politeness and does not change tense itself. This kind of makes it sound like "[the fact] is, it was fun" (both a present and past tense in one sentence).
でした only comes into play when the i-adjective cannot make a paste tense of itself, as happens with formal negatives. E.g. when instead of saying 楽しくないです you use 楽しくはありません, the latter can't be conjugated any further. So an average polite 楽しくなかったです becomes a highly formal 楽しくはありませんでした.