When is the next time the FX network or any other network will air the
movie "Halloween:H20".

Hello arkin,

I am unsure when Halloween:H2O will be aired next, but you might want
to consider purchasing the vhs or dvd from ebay. The prices are very
low and then you could watch it whenever you want to.

If interested, take a look at both of these URLs:

This search is for Halloween H20 (the number zero at the end, not the
letter 'o'. This is a mispelling but people have listed it like this
and the prices seem lower because of the spelling error).
http://search.ebay.com/search/search.dll?cgiurl=http%3A%2F%2Fcgi.ebay.com%2Fws%2F&krd=1&from=R8&MfcISAPICommand=GetResult&ht=1&SortProperty=MetaEndSort&query=halloween+h20

This search is for Halloween H2O (the letter 'O', not the number
zero).
http://search.ebay.com/search/search.dll?cgiurl=http%3A%2F%2Fcgi.ebay.com%2Fws%2F&krd=1&from=R8&MfcISAPICommand=GetResult&ht=1&SortProperty=MetaEndSort&query=halloween+h2O

If you are happy that this response answers your question, please let
me know in a comment and I can submit it as the official response. You
can of course leave the question open so that another research can
find the next time Halloween:H2O is aired on a network.

All the best,

tisme-ga