It was October _when__ the People’s Republic\x0b of China was foundedIt was October __when_ the People’s Republic\x0b of China was foundedIt was in October__that_ the People’s Republic\x0b of China was founded.为什么

It was October _when__ the People’s Republic\x0b of China was foundedIt was October __when_ the People’s Republic\x0b of China was foundedIt was in October__that_ the People’s Republic\x0b of China was founded.为什么
It was October _when__ the People’s Republic\x0b of China was founded
It was October __when_ the People’s Republic\x0b of China was founded
It was in October__that_ the People’s Republic\x0b of China was founded.
为什么

It was October _when__ the People’s Republic\x0b of China was foundedIt was October __when_ the People’s Republic\x0b of China was foundedIt was in October__that_ the People’s Republic\x0b of China was founded.为什么
第一个句子：It was October when ...这里的 when 引导定语从句,修饰 October,全句直译是：那是中华人民共和国成立的十月.（当然,此句需要意译才会更通顺）.
第二个句子：It was in October that ...是一个 it was...that 的强调句型,它的陈述句是 The People's Republic of China was founded in October. 改成强调句型后不能漏掉陈述句中的任何一个成分,因此需要完整的状语 in October.

前一个是定语从句，修饰时间用关系词when。后一个是强调句，句型是It is/was …that …，这里是介词短语。

when
这是定语从句,做时间状语

中华人民共和国成立于十月
这里是一个强调句，It was ~~~when 的句型