Im gonna share based on my current knowledge when it comes to celebrating other's festival, forgive me for any mistakes i've made in my statement. Even though Rasulullah S.A.W never said celebrating halloween is haram but we can ask ulama' or ustaz or ustazah(teachers about islam) or those who learn deeply about islam because insyaallah Allah will share the knowledge through them.
First we need to know what halloween is about. The ancient Celtic (Irish/Scottish/Welsh) festival called Samhain is
considered by most historians and scholars to be the predecessor of what
is now Hallow’een. Samhain was the New Year’s day of the pagan Celts. It
was also the Day of the Dead, a time when it was believed that the souls
of those who had died during the year were allowed access into the “land
of the dead”. Many traditional beliefs and customs associated with
Samhain continue to be practiced today on the 31st of October. Most
notable of these customs are the practice of leaving offerings of food
and drink (now candy) to masked and costumed revelers, and the lighting
of bonfires. Elements of this festival were incorporated into the
Christian festival of All Hallow’s Eve, or Hallow-Even, the night
preceding All Saint’s (Hallows’) Day. It is the glossing of the name
Hallow- Even that has given us the name of Hallow’een. Until recent
times in some parts of Europe, it was believed that on this night the
dead walked amongst them, and that witches and warlocks flew in their
midst. In preparation for this, bonfires were built to ward off these
malevolent spirits.
Based from above, we can see that celebrating halloween is syirk and those who celebrate with believing it are clearly haram.
Next, is the candy and costume part, yes it is true the candy and costume may be halal, but where did you get it and why? from halloween events which following it can be makruh(have a chance to fall to haram) because of getting it from a syirk event.
Why do you wanted to get something close to a thing that is haram? There are no other places to get candies? Or any other time? Because it is free? How if it's actually another test by Allah s.w.t on us? Whether we can reject the candy and remember that Allah can give a lot more reward than just candy on other's celebration? Wallahualam..
That's all for now, apologize again of all my weakness and short of answer because it is based on my current knowledge. All the good ones are from allah and the weak statement are from my own weakness. Please do correct me may we all can achieve new good knowlegde from time to time. Assalamualaikum