Its an excellent question and I want to answer it in two different ways. The first is how to identify lead volume by market and time/day and there is a way we accomplish that relatively easily. We use a unique tracking number in each market and the leads are all time stamped. That data is then graphed out automatically so we can see the specific lead trends by market/day/time/etc., use that data via our Customer Success Managers to determine if we need to make changes. You're right though, that there isn't a way to see actual traffic. Again, this is Craigslist M.O., they don't share that type of information at all. Much like everything else, trial and error is the answer here.
The more complicated part is the "magic time" theory of posting. We dedicated a fairly long development sprint to "time slicing" and campaigns and identifying the best days and times to post. What we found was that, in most cases, posting at a roughly peak time (like 10 AM) is most effective and that creating windows didn't really have that much impact (particularly compared to the rabbit hole that you can go into trying to find your "magic time" to post). We also found that in general it was best to post every day, not taking any days off.
I've seen the data, I 100% agree with my team that it supports this...but...I do think in the handful of really large markets in the US, this becomes more complex. Places like the Pacific NW, where people overpost in huge volume, posting strategies that take into account time of day and day of week may be more important and could have a small (single digit percentage) impact on lead volume...but small impacts on lead volume aren't without merit.