Based on funding mechanisms, the showdown could come either in September or December — or both:

Officials at both ends of Pennsylvania Avenue who are up to their necks in tax reform think passage probably doesn't happen until early next year.

who are up to their necks in tax reform think passage probably doesn't happen until early next year. So a September shutdown could be better for tax reform than a Christmas shutdown, because it would allow conservatives and Trump to get it out of their system.

Democrats feel certain they have Trump boxed in, and see no reason to compromise/help:

Trump is at war with Senate Leader McConnell and several other Republicans, complicating communications and compromising trust.

with Senate Leader McConnell and several other Republicans, complicating communications and compromising trust. Congressional leadership doesn't want a shutdown and can pass the fall bare necessities — continuing resolution / debt ceiling / Children's Health Insurance Program extension — using mostly Democratic votes.

doesn't want a shutdown and can pass the fall bare necessities — continuing resolution / debt ceiling / Children's Health Insurance Program extension — using mostly Democratic votes. But the Freedom Caucus will hammer Speaker Ryan for doing so, and conservatives in the Senate will hammer Leader McConnell.

Be smart: With the departure of Steve Bannon, Trump is surrounded more and more by conventional/mainstream folks, which could actually make him feel more compelled to buck them.