New passes are visible on the 1st day of the current month. Available passes for the next three months can then be seen.
For example, on October 1, available passes for the months of October, November, and December are visible. Although the passes for October and November may have already been reserved, new passes will appear for December.
If passes for the current month are not available you can still check for those periodically too, as other users sometimes cancel their Discover & Go reservation, thus making a pass available again.