In Chinese, time is often conceptualized with vertical or directional metaphors. The past is “上” (up/above), and the future is “下” (down/below). You can visualize this as standing in a river (the river of time): to see what just passed, you look “upstream” (上); to see what's coming, you look “downstream” (下). This contrasts with the common Western metaphor of a linear timeline where the past is “behind” and the future is “ahead.” While both cultures see time as a sequence, the Chinese directional “上/下” for “last/next” is a unique and consistent linguistic feature. Understanding this simple concept helps unlock not just “上个,” but also its opposite “下个” (xià ge - next) and related terms. It's a small but fundamental window into how the language structures its worldview.
“上个” is extremely common in daily conversation and informal writing. It's almost always followed by a measure word and a noun, or just a noun if that noun can also function as a measure word (like in `上星期`).
The term is neutral in tone and can be used in almost any context, from chatting with friends to discussing business reports.