1、一般情况下是不加的。在有特定限制的时候，会加介词。
2、today和tomorrow等一系列表示时间的词可以做名词，也可以是副词，加不加on , in等副词，要看today和tomorrow等词前面有无the定冠词。如果有，则需要加。
拓展资料
today
1、How are you feeling today?
你今天感觉怎么样？
2、The United States is in a serious recession today
当今美国正处于严重的衰退时期。
3、I wanted him to come with us today, but he couldn't.
我想让他今天和我们一起来，但他来不了。
4、He thinks pop music today is as exciting as it's ever been.
他认为现今的流行音乐与以往一样令人兴奋。
5、This was a prologue to today's bloodless revolution.
这拉开了当今不流血革命的序幕。
tomorrow
1、Bye, see you tomorrow
再见，明天见。
2、What is education going to look like tomorrow?
未来的教育会是什么样子？
3、The first official results will be announced tomorrow.
第一批官方结果将于明天宣布。
4、What about going out with me tomorrow?
明天和我一起出去怎么样？
5、So you won't go tomorrow, eh?
啊，那么你明天不去了？