You mean I have to wait?
Just kiding. I figured I'd need to wait until official dates are announced. I just thought the experts here might have some theories.
There are a few scenarios that could take place this year, during Fall/Halloween Time. Some of this speculation is based on previous rumors that trickled out last year, and some of it is just based on deductive reasoning when putting all the pieces of the puzzle together.
One scenario is that the current
Disneyland Forever fireworks could have some sort of extended schedule, running on nights when fireworks might not ordinarily be shown in the off season (which is what happened in 2009, when Halloween Screams debuted for the general public and ran every night, all Halloween season long, with the exception of any nights that were affected by weather) -- simply because these are special fireworks created for a special year, and Disney may want to make them available to as many people as possible, for as long as possible, in 2015. I don't know if they'd have to get approval from the City of Anaheim to run the fireworks on nights when there ordinarily would not be any fireworks, but there must have been some sort of arrangement made in 2009. Presumably, Paint the Night will run on most non-party nights for the rest of the year as well.
Another scenario is that the Halloween Time season could start on a day other than a Friday this year. Instead of September 11th (only 4 days after Labor Day), or September 18th (which seems too late), the season could start anywhere between Saturday, 9/12 and maybe Wednesday, 9/16. They could even do something cute -- because this is 20
15 -- and start the season on 9/15/15. ( Interestingly, WDW's Halloween parties seem to be getting a later start this year than they have in some other years. Wasn't it just a year or two ago when their Halloween party started on September 1st? This year,
MNSSHP is starting on 9/15/15.)
Because Halloween falls on a Saturday, and 10/31 would normally be the end date of Halloween Time, it's possible that the season could extend through Sunday, 11/1 -- especially if there is not going to be a Christmas parade taping on Main Street several days later (no Christmas parade this year).
Another scenario is that, in keeping with the "15" theme, there could be 15 Halloween parties this year (up from 14 last year)... or there could be even more than that. One of the rumors from MiceChat/MiceAge a while back was that Disney wants to add more nights to Disneyland's Halloween parties, and start them earlier in September than they would normally start them.
So, while a typical MHP schedule would probably have MHP kicking off on Friday, 9/25, if the rumors are true the party could begin a week or week and half earlier than usual, and include more nights.
Of course, then we have the rumor of MHP possibly moving over to DCA. While I think that this would most certainly be a good year to do that, there have not been any recent 2015 rumors of this happening -- which leads me to believe that it may have been a consideration at one time, and is now off the table.
In fact, yesterday I was trying to find the 2014 thread on MiceChat where a forum member first posted that he/she had overheard some CMs saying that the party would be moving to DCA this year (it was originally posted as "News" until the MiceChatters complained and said that it should be labeled as a "Rumor"). That thread was active for at least a few weeks in October-November last year, and I can no longer find it!

I searched and searched, using all of the keywords I thought would pull it up, and I am normally very good at locating old posts and old threads.... and it is nowhere to be found. I have no idea what happened to it.
In that same thread, if I recall correctly, another person said that the CMs who trained for MHP last year were told that big changes would be coming to MHP in 2015. So that is another scenario -- MHP location aside, it is possible that the events included in MHP could change somehow too.
All of that said, I don't think there will be a party on Saturday, September 19th. Disneyland seems to like to avoid Saturdays and Sundays for parties whenever possible, and the only reason there will be at least one Saturday in the schedule this year is because Halloween night is a Saturday. They can't skip a Halloween night party. But... if the rumors pan out, the party might not be in DL and could be in DCA instead, in which case it wouldn't interfere with the 60th's entertainment.
I think there would definitely be both Disneyland Forever and Paint the Night on Sunday, 9/20.
While certain elements of the 60th anniversary are going to last all year and into next year, Disney is not going to completely turn its back on Halloween Time -- and especially not on the holiday season -- when it is such a popular time of year for people to visit. Some aspects of the season could be impacted, but there are not a whole lot of Halloween decorations to begin with (outside of Main Street and Frontierland), so I don't expect the pumpkins to take a back seat to blue bunting or signs that say "D" or "60" or that have diamond shapes on them. People who are Halloween Time regulars will want to see the giant Mickey pumpkin in Town Square, 60th anniversary or not. If anything, Disney will probably find a way to integrate the décor and make everything work somehow.
I'm sure some of the 60th-specific treats will still be around, but again... Halloween Time is very popular, so there will be Halloween treats too.