这个句子没错，什么也不缺。

这是动词be 的一个特殊用法，意思相当于happen, take place, be held 等意思。

其主语往往是表示“事件”的名词，如：meeting, talk, lecture, report, party, discussion, match, game, party, concert, play, accident, eruption, event 等。因此，上句译成汉语应该是：“聚会（派对）是在8月份的一个星期天举办的。” 又如：

---When was her birthday party? ---It was the night before last. 她的生日派对（聚会）是何时举办的？前天晚上。(was 等于was held)

Our school sports meet will be next week. 我们学校的运动会将在下周举行。(be 等于take place)

It was reported that the accident was on Tuesday and five people were killed. 据报道事故发生在星期二，有五人丧生。(was 等于happened)

This is when small amounts of gases in the atmosphere, like carbon dioxide, methane and water vapor, trap heat from the sun and therefore warm the earth. (选修6第四单元，Reading) 这种现象（温室效应）发生在大气层中少量的气体，如二氧化碳，甲烷，水蒸气等，吸收太阳的热量，因而使地球变暖的时候。(is 等于takes place)

下列两句很有意思，一字之差，但意思相差甚远。请比较：

It was Sunday. We went out for a picnic.

It was on Sunday. We stayed at home and didn’t know anything about the accident.

有人把It was Sunday.和It was on Sunday.混为一谈，认为意思没什么区别。其实，这两个句子的含义大不一样。前者译为“时间是星期天”，It指时间；后者译为“事情发生在星期天”，It 指某个事件。

一般说来，be 表示“发生”意义时，除了它的主语表示“事件”概念外，还往往有一时间状语跟它连用。