Each company on iSS has a separate contract with each of their clients, so there is not a single release date for iSS. Moreover, iSS administrators are unlikely privy to that information.
If you are looking do more work than what iSS offers in your area, have you looked outside of iSS for mystery shopping opportunities.
Like MFJohnston said, each MSP has multiple clients, with multiple shops of their own, and they all get posted when the clients and MSP determine. As a general rule of thumb, monthly shops are posted about 1 week before the start of the next month, and weekly shops are often posted at the beginning or in the middle of the week, for next week. This is done so the shoppers would have time to see them and assign them. Of course, there are always exceptions.
After a while, you will start to notice patterns in these weekly and monthly shops, as shop for the same clients are often posted at regular intervals.
You can always contact each MSP and inquire about their specific shops and when can you expect them to be posted.