IMO the best thing to do would be to find the time zones of your biggest player groups and post when most people would be available. Biggest group is probably North America, not sure what parts of Europe/anywhere else have a big showing for B101.
I'd then consider what times people are usually free. Between school/work/what have you, many people aren't going to be available in the time slot from around 7 AM to 5 (maybe even 6) PM. So try to think about posting around that window. A lot of people are going to eat around 6/7, so getting the thread up around 8 or 9 would, to me, be ideal. Not too late for even those of us with bedtimes or early mornings, and not so early that it would conflict with most activities or what have you.
And post on a weekend. That frees up many more time slots, for people working 9-5 M-F or for those with school. There are, of course, people who work weekends, so picking 2 different days might be nice, and setting the time (as stated previously) somewhat later in the day may be beneficial.
I think the TL;DR for this is that if you were to post for a single time, I'd pick 9 or 10 PM EST which isn't super late for the East Coasters, and also gives those of us on PST a time of 6 or 7, which is also fairly friendly towards anyone working or schooling. Since the threads fill up fairly quickly I don't think it's an issue to have someone committed to staying up all the way to 11 PM EST (so late!) in order to close the thread in a timely manner.
Do a similar thought process for your biggest group of non-NA users and you've got yourself a pair of very accessible sign up times.