What’s the best day to send a newsletter?

It depends on your audience. I'd post a survey asking your readers for their preferences.

Personally I'd tell you Friday, late afternoon. I like to keep my inbox clean, and during the week, with work and such and no time in my hands, I often defer reading newsletter to a later time (which guess what, it's usually the weekend). So, sending it on fridays, late afternoon, I think you get to be at the top of people's inboxes when the weekend comes, and with it, more time to sit back and relax while reading a good old interesting newsletter!