Really hard to hear the difference between De and Det. Both would have made sense here.
I think the e in de sounds like in the English word me, while det sounds like the English word day. It took me a while to start hearing this as well, or maybe the text-to-speech engine just got better over time and added this nuance at some point.
I think this evening would translate to denne aften. But it's a slight difference.
As of Feb. 20, 2015, "They begin this evening" is suggested as another correct answer.
I've seen from other translations that using words indicating a future time in combination with a present tense verb basically equates to the future tense in other languages. Is there anything incorrect about the following answer? "They will begin tonight."