Go to your channel analytics, audience. Then you will see something like this:
View attachment 13229
In my case, the Purple bars are DARK around Tuesday 6pm. So, I always post on this time frame. It might be different for you. This is actually based on my niche. Your niche is different. I hope that helps.
I follow a successful YouTuber, Robert Benjaman, I watched his video recently. He stated that these are not just subscribers, but the people on YouTube of your niche."When your viewers are on YouTube
This report shows you when your viewers are online across YouTube in the last 28 days. You can use it to help build your community, understand when to schedule a Premiere, or to plan your next live stream."
Does this report include all viewers or just your subscribers? I couldn't find that answer anywhere. Thanks.