Dammit there's a show on the 20th? Well crap there goes free parking for the game at Nissan Stadium. But yeah, anyway, welcome to town. Going from Brentwood to downtown isn't a huge hassle but I believe there's a little bit of northbound traffic on 65 because there are quite a few people who commute to work in Brentwood, so then they leave and go home (I used to do this). If you want to be downtown by 6 for example I'd probably leave around 5 just to be safe in case there's any issues on the road. But if you wanna get there early and do things before the show then you can leave whenever and there's rarely afternoon traffic.
Typically rush hour runs between 3:30 and 6:30 depending on the parts of town you're in with the peak times probably going from 4:30 to 6. Again, depending on the area. 440 is a bad road to be on. I-24 is a terrible road to be on. But that's generally outbound traffic so I don't know how much you'll be affected. However, side note, as I kinda mentioned earlier our game 4 is on the 20th so there may be more traffic coming from Brentwood and south of Brentwood heading towards downtown for the game. Game time hasn't been decided yet but with shows and a playoff game, traffic may/probably will increase. Depending on where you live now, Nashville traffic will either seem like nothing or be a complete annoyance. Unless you're going against the flow of it. They say 80 to 100 people move to Nashville every day, so traffic is quite literally getting worse by the week.