You can get into the park with your MNSSHP ticket at 4pm.
Also, if you don't have enough room to bring a full on costume, think about going disneybound style. I wore a red & white polka dots dress and my sparkly Minnie Mouse ears when I went to MNSSHP in 2014. During my vacation last year, I went during
MVMCP, and on that trip, I brought some cute outfits to DisneyBound with just clothes I had in my closet and did a little nod to Belle, Ariel, and Snow White. For Belle, I had a yellow skirt and yellow shirt, Lumiere charm bracelet, red purse, and a red rose necklace. Ariel, I had a green skirt, purple shirt, dinglehopper / mermaid charm bracelet and red purse. For Snow White, the yellow skirt again, blue top, red purse, Minnie Mouse ears and a Snow White themed charm bracelet.
ETA in photos
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Hope you have a great time! I really enjoyed MNSSHP, even though it was busier than I thought it'd be. Halloween is my favorite holiday though, so I may be biased. Loved the parade with the Grave Diggers and Haunted Mansion dancers. Hallowishes was awesome, too. I didn't get any trick or treat candy, not my thing, so idk anything about that. The parade, fireworks and rides, as well as the atmosphere, (looking at all the costumes) were my focus. I only met with Jack Sparrow. Only waited about ten - fifteen minutes. Most of the character's lines were very long. Also, Main Street and the hub, tends to be pretty crowded during these parties. So, depending on what you decide to do, it may feel more or less crowded to you.