It is pretty clear that you are picking up wireless inteference from another source near you.
My first thought.....a wireless camera/security system near you is switching on each day at 8:30 PM and it is interfering with your network.
Another neighbor's wireless is set to download or upload a large amount of data each night starting at 8:30 PM, and it is likewise interfering with your network.
You or someone near you may have a child going to bed about 8:30 each night and the baby monitor is set to go on at this time and it is interfering with your network.
How to identify these sources is one issue. What if anything you can do once you identify them is another.
You may need to get an IT pro out with professional scanning and diagnostic equipment to help narrow down the possibilities.