I want to add about the word's position in this sentence about this stupid fish.
Did you buy fish for tonight dinner? - better to say "Ты купила рыбу сегодня на ужин?" - doesn't matter when the bought this fish, may be yesterday, but we plan to eat it tonight.
While "Ты купила сегодня рыбу на ужин?" or "Ты сегодня купила рыбу на ужин?" means that the fish should have been bought today. "Сегодняшний ужин" for my ear sounds awkward. The better equivalent would be "сегодня на ужин".