Indeed, and it drives me nuts.
I loath the retail trend of 'pushing the Holidays' so far in advance that they encroach on other holidays.
Perfect example being Halloween - drives my nuts to see X-mas stuff in stores in September!
One holiday at a time, people!
But I digress.....back on topic.
If seeing Disneyland's version of 'Fantasmic!!' Is a must ( and I highly recommend it ) then I would wait to see if they announce the schedule far enough in advance so you can adjust your dates.
As GiveMeTheMusic noted, the Show normally does not run on Monday nights in September, but this year might be an exception if the new edition is packing in the crowds.
Typically, the Park Schedules seem to go up on the official Disneyland website about a month before ...hopefully that won't be too far along for you not to be able to make your plans.
I have a feeling that if the new edition of the Show is a real smash, they might extend the nightly showings.
No way of really knowing this early...we will all just have to wait and see.
-