Hi,
we usually aim for Thu but it often slips to Fri..Generally there is no big difference because we wouldn't fix anything on Friday as the fixes will be in next week beta. The weekend is important for us to collect feedback, both here and from our telemetry. If we would release on Mon, then we would have data on Wed and then almost no time to fix them as the beta branch is usually created 3 days before beta release...
Regarding the quality....It's not comparable to state 2 or 3 years ago, when we were releasing 3 beta versions per quarter and the first 2 were usually in bad shape..
Yes there are still some minor issues not fixed between the betas but the betas aren't crashing or causing BSODs..