Halloween is generally associated with death and the underworld.
It’s possible you have been thinking about these themes lately and it’s filtered into your dreams. If you have suffered a death in the family or have been thinking about the existence of a hell- this could be why you dreamed about Halloween.
This is also a time of year to dress up and take on a new persona so perhaps you want to be someone else. You might envy someone in your waking life for what they have or how they act. Consider what aspect of them you desire and ask yourself why you covet it so much.
On a more positive note, dressing up can make people more confident and outgoing- it gives them the freedom to express themselves. It’s possible you have felt this way lately- you can finally be your authentic self.
On the other hand- are you trying to hide yourself beneath a mask? What are you afraid of? Why are you hiding? It might be time to let others know who the real you is. If they don’t like it- that is their issue, not yours.
If Halloween was a significant date in your calendar as child- perhaps you miss the things associated with this celebration. Seeing your friends, having fun, being mischevious, the sense of community- whatever it might be- perhaps you need to bring these things into the present and embrace them again.
If you were the only one dressed in a Halloween costume- be careful who you trust for some people might be out to deceive or embarrass you.
Halloween is also a time for scares- perhaps the thought of doing something in your waking hours terrifies you. You might be overwhelmed by fear. Maybe you need to do whatever is required to conquer your fear. Or maybe you need to accept that you are too afraid to do whatever it is and find something you are more comfortable with.