Do you know if Thursday night is "back up night" (or some other maintenance) for your Exchange Server?
I dont think either of those, unless real downtime was involved, would effect you receiving email.
Do you know the time frame, from 7pm to 10pm or 5pm to 11pm, etc? If you can get a solid timeframe ask your BES admin if they are doing any sort of special work during that time period.
I have the same BB as you and can recieve email on Thursday nights.