"I know what you did in July."
Translation:Jag vet vad du gjorde i juli.
att is commonly used where you would put the word "that" into a sentence. Example: I know that you did it in July ... Jag vet at du gjorde det i juli. In the above example, no "att" is necessary. That's my understanding of it anyway. It's easy for me, knowing Spanish, where the particle word "que" serves the same function as "att" here.