Hi Manic,
People working at night where the public can enter need some basic form of security and emergency response procedures and related training. As you say, Halloween night adds a few elements because people can enter dressed up in costumes (is it a robber or a bonafide trick or treater?) or because some people may be intoxicated or high on that night, which many tend to party on, and mingle in public.
If I am correct, in accordance with provincial or municipal law, employers must make all reasonable efforts to protect their employees.
If you feel comfortable doing so, one thing you could do is ask your employer if it's possible to provide you with an assistant that night (if they do inventory at night, could they do it on Halloween night?) and explain that you have concerns about your security on Halloween night.
If an assistant can't be provided, could your employer ask police to patrol more often? Perhaps offering free coffee if they drop in? Or could your employer drop by a few times?
I would also pre-program 911 on the store telephone (this way, if you can't talk, the 911 operator can determine where the call is coming from). If that's not possible, I'd pre-program my cell phone and have 911 appear on my display, ready to dial at the push of a button.
Another possible option would be to ask other store employees, assuming there are any, if they would mind trading shifts with you.
Another one, and this may only be a possible future option due to time restrictions, would be to install a concealed panic button connected to the store's alarm system, assuming there is one, somewhere where the cashier can press it during an emergency, some place hidden from view.
I hope this helps you and that it goes smoothly and uneventful on Halloween for you.
Johnny