yesterday是后面的which从句的先行词。去掉杂七杂八的成分，就是today was yesterday，yesterday是你这句话的宾语，所以放在句尾就出现语法错误了
YestErday在主语面前是不需要加逗号的 这是因为时间状语可以直接和主语相接，不需要逗号，不过有些剧是根据个人意愿，也可以加逗号，个人见解，仅供参考
后面可以加逗号，也可以不加，例如：
1、Oh yeah? I wasn't born yesterday， you know.是吗我可不是三岁的小孩子，你知道的。
2、The government yesterday announced to the media plans to create a million new jobs.政府在昨天向媒体宣布了创造一百万个新工作的计划。
3、There were masses of people in the shops yesterday.昨天商店里人如潮涌。
