My taxes are complicated:
...  I'm self employed,
...  I have my own business,
...  I'm partners in two other business (two different partners),
...  I file taxes in my home state & in another state,
...  I have a handful of rental properties (and I'll never understand depreciation),
...  Hell has a place for estimated taxes,
...  (This year, I've had to deal with estate taxes & their complications),
...  and so forth.

As a result, I have 6-8 returns I have to file, and they are confusing as crap.

So, it's a big deal to me to finish the taxes.  It's lunchtime right now, and everything is ready to go.  Everything that needs to be mailed in have been printed along with the associated envelopes.  I need only write a few checks, stuff the envelopes, and I'm done.  It's a big, big relief to get done.

So, what do you do to celebrate the completion of this awful task?

This year, I bought photography equipment with my (sweet) return.

It was celebratory.

The last couple years completing my taxes has not been cause for celebration.

If we get a refund we celebrate by putting the refund in the bank...

Depends if the result is worthy of celebration 

i am getting a good return back.. i might treat myself to a nice tattoo or maybe hired a nice model for a shoot.