我们通常会说今天，今日多用于地方方言中，如果广东话，就经常用到“今日”。如果你学普通话，只用“今天” 就可以。希望可以帮到你。

They are similar,'今天'is more common to say. we use '今日' in writing.

今天： 1、说话时的这一天;本日[today]，今天的作业 2、用在指一天之中某个时候的名词前面,表示指的是今天的某个时候[this day]，今天早晨起得太早了！ 3、此刻,现在;当前 [now;at the present]， 在今天再去讨论这个大家公所周知的问题已经没有必要了。 今日： 1、本日，今天[today, this day] ，今日事今日毕 usually this meaning of 今日 = the 1st meaning of 今天 2、目前，现在 [now,at the present] 今日的事情还没有做完。 usually this meaning of 今日 = the 3rd meaning of 今天 most of time, 今天 is used in both our spoken and written occasions. 今日，however, is most used in written occasions, especially in some proverbs ,such as 今日事今日毕。

when u not so sure about which to use , just say 今天，because 今日is more like in formel articles

用法没有什么区别，都是today的意思。同样表示today的还有今朝、朞日儿等