On October 31, for no other reason than it falls on a Monday, I leave my wife.
Your opening sentence is still rather clunky, especially the 10 words following the date. Try reading it out loud. It suggests he's only leaving his wife because it's a Monday when I guess he has other reasons for doing so.
How about
On Monday, October 31, I leave my wife. Mondays always make for a tidy start.
Then we have another sentence that tends to run on and on. Maybe it's intentional, to convey the woolly-mindedness of your MC, but it's not great reading:
I’ll be gone two nights, I tell her readily enough but hold back on for good, forever, words I’ve been mulling over for a while but have done nothing about.
Does this achieve anything? We already know he's not included the words 'for good' and 'forever'. It seems unnecessary, I feel.
And finally:
Later the irony of the date, Halloween, will not be lost on me. I should have picked a better day.
That's OK, but in danger of coming across a little contrived because you make such a big deal of the date.
As usual, just one opinion. Use or lose.
H3K