I'd like some clarification on some things Unusual hat related.

Some Unusual effects are "Halloween themed", for ex. Secret to Everybody, Stormy 13th, Knifestorm, etc.

Some Unusual hats are "Halloween themed", for ex., Crone's Dome, Wraith Wrap

Are hats and effects season exclusive? Meaning, can I only get a Halloween effect during Halloween? Or does it require a certain crate? Is it based on contents of crate? Is it a specific period of full moon cycle?

Some clarification would help, thanks.