Sometimes they might cancel them depending on the weather conditions. The winds might blow them the wrong direction. And make it dangerous.
Even though it is almost always busy at Disneyland they don't have fireworks during the week in the slow season and during Halloween time they only have them of the night of Mickey's Halloween Party which requires a special event ticket, if they day you are going is considered a value day don't expect fireworks. You can check the entertainment section of Disneyland's website to check to see if they are going to have fireworks.
I don't know but they usually have a fire works show everynight
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