This is the 468th most frequent Japanese word.
昨
"昨" means "yesterday" or "previous."
Here, 昨夜 (さくや) uses 昨 to refer to a past time (last night).
昨夜はたくさん勉強しました。
Last night, I studied a lot.
In 昨日 (きのう), 昨 denotes 'yesterday' to describe the specific day before the current day.
昨日の天気は晴れでした。
Yesterday's weather was clear.
In 昨春 (さくしゅん), 昨 indicates the previous occurrence of the season 'spring' (last spring).