Soil temp, not air temp.

So this is the first day?

Is anything greening up?

Has it rained recently? Because all grass seed needs fairly consistent soil surface moisture to germinate. If you can allow the soil surface to dry out completely before you water (and I'm talking about several weeks at that low temperature), then you should not have crabgrass issues.

Is your turf fairly dense? Dense turf will almost always stop crabgrass because crabgrass seedlings need plenty of sunlight. If the grass is tall and dense, then any seedlings which sprout will not get enough sunlight to survive.

If you have moist soil and thin turf, I would put the Halts down now.

I have dense turf every year but when july and August comes and we get moderate drought , the grass withers and the crabgrass takes over.
Again, Should it be put down before the three 50+ degree days, during, or after?

As you mentioned and as dchall emphasized it is soil temp rather than air temperatures. Best guide is when the forsythia bloom.
To answer your question, Halts is a pre-emergent. Pre-emergents are applied "before."
Most pre-emergents protect for 60-90 days (shorter for siduron) If you wont be seeding in the Fall you can put down another pre-emergent application in late June (Best to use a different AI, like prodiamine or dithiopyr.

Maybe you did not read my reply all the way to the end. I would put it down now.

Ok I'm not sure why every one thinks I don't understand soil and air temp as I said "soil" temp in my original post.
I put the preventer down now.
Scotts bag says before grass is mowed 3rd or 4th time but that seems too late.
I realize crabgrass does not come up at one time together but at different times through the Spring.
Hopefully I have got it down at the right time to prevent the most of it.
Even though I get a lot of it in some areas later in the summer.

This year I put it down before my first mow because of the weirdly cold spring time. I think it's more important the soil temperature and they say after the 3rd mow cuz generally that's when the soil temperature is right.