We say 上个星期, 上个月 and 去年 why not 上个年?
We sometimes use 上一年 which means the same with 去年.And we say 上个星期，上个月，also means 上一个星期，上一个月，and we ignore 一 for easy expressions. At the same time，we ignore the 个 in 上一个年 for the same reason. With time passing by, we use them for regulations. As for 去年，昨日，in the ancient time, we use 去，昨 mean past or 过去的，nowadays we remain those words. In my memory, we alse use 去月 means past month.