I do not believe the 24 Hour Event reached capacity. It was crowded from the afternoon on through to about 1am. I believe (and someone can verify) that after the crowds for the Leap Year 24 Hour the CMs were not allowed to the Monsters 24 Hour unless they used a "pass". This may have helped.
Last year's party was not announced anywhere near this early to allow for people to plan to attend. They are really advertising it in relation to the release of Malicifent with villains and that is appealing to another whole level of audience, such as those who attended the Unleash the Villains - a free party!
Unleash the Villains was a nightmare!!! We arrived early in day because we knew that it was going to be packed. Apparently Disney did not get the memo.
The theme, the permitting of costumes, all the extras with no party ticket brought in thousands and thousands of locals and non-working CMs. In short the park was grossly too small for the event (MK is better), there were drunks (MK no alcohol), the events were poorly planned and setup, and it became a "unsafe" situation.
I think this upcoming 24 Hour Event is going to be a cross between the two because not only are they open 24 Hours but they gave permission for costumes without a party ticket. This means many who did Unleash and enjoy villains, cosplay, steampunk, anime, gothic etc dress will see this a great opportunity for a fun costume all night party. The only positive is there is no alcohol in MK.
I would not plan the attendance based on just last years, I would add more people. Permitting costumes when they normally are not allowed, potentially changes everything! I would think once you hit the end of day work hours 4 pm .... getting into the park, especially by car is going to be a challenge.
If I were able to go again this year

sad: have to go to a wedding) I would be getting there no later than lunch time and planning to stay put. I would also get myself away for Main Street until at least midnight. We did that last year and avoided most crowds as after 1-2 am many had left. We stayed until closing and the end was really the best part of the night!!
Have fun!