You use yesterday to refer to the day before today.
She left yesterday.
她昨天离开了。
yesterdays
Yesterday is the day before today.
In yesterday's games, Switzerland beat the United States two to one.
在昨天的比赛中,瑞士队2比1打败了美国队。
You can refer to the past, especially the recent past, as yesterday.
The worker of today is different from the worker of yesterday.
如今的工人跟过去的工人不一样了。