Are passes for all attractions always available on the first of each month?

No.

For example: some attractions only have seasonally passes, or provide passes for a specific event, such as a special exhibit, a limited run performance, or a specific concert date. 

In other cases, there is sometimes a pause in pass donations at the end of an attraction's fiscal year, or during a black-out period they have requested, due to facility maintenance, or other business concerns.

As mentioned elsewhere, any available passes for the next three months (the current calendar month and the following two months) can be seen. BUT, when new passes become visible on the 1st day of the current month, those passes are ONLY for the third month which has just appeared on the calendar.

For example, on October 1, available passes for the months of October, November, and December are visible. Although the existing passes for October and November may or may not have already been reserved, new passes will appear for December.