昔 is literary/classical Chinese. In modern speech, use 以前 (yǐqián) or 过去 (guòqù) for 'in the past'. 昔 survives in set phrases like 今昔 (then and now), 昔日 (former days), 往昔 (the past).
memory
昔 looks like 廿 (twenty) on top of 日 (day/sun) — picture twenty suns ago, deep in the past.
