They use both: tonight and today evening. However tonight seems to be used when it is later.
so the phrase is saying something like... - today "eveningly" (if there were such an adverb in English lol.)
Sorry for being such a nerd, I'm trying to understand the grammar behind all these phrases. I got up to 20 lessons a month ago and realised I still had no clue so I realised I had to go back and learn a lot of grammar and now I'm obsessed with it.