Here in Boise, ID, zone 6, I put mine out by the 1st of March. They usually emerge within a couple of weeks. It really depends on when your fruit trees start blooming. If it's warm enough for the trees to bloom, it's warm enough for the Masons to emerge, generally about 50 degrees.
Happy Gardening!!!!!!!!!!! Tom

In early spring about 2 weeks before your trees start to bloom set out the nesting blocks with bees.

Here is a link that might be useful: Mason Bee

I agree that mason bees should be placed in their emergence location at least one week prior to fruit bloom. Male bees need to emerge first, then females, and they need at least a day or 2 to mate and get their acts together before they will be competant to pollinate your trees. Don't move them once they've started to emerge. An outstanding site for information about solitary bees is Pollinator Paradise (Dr. Karen Strickler), linked below.

Here is a link that might be useful: Pollinator Paradise

Well...I didn't want to put them too learly, but some trees here are already in bloom. Mostly pluotsa dn low-chill apples. I set them out today, let's see how they go this year.....Most peaches and nectarines are about a week away from full bloom....so I hope to have timed it right !

You should be right on. Those trees will be in bloom for a while...the timing does not have to be exact.
How many tubes do you have to put out? If you have enough you can extend the season a bit by putting some out 2-3 weeks after the first batch.
Happy Gardening!!! Tom

Tom- Exactly what I did....I put out 12 tubes first....Then I've got another 15 tubes to put out, in about 2-3 weeks. I've also got some Osmia Californica, that I'm putting out with the batch in 2-3 weeks. Let's see what we get...EZ

Took a pic of them, so that I can see any progress.....

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Looks great. Three years ago I started with 3 tubes of masons and got 20 by the time they were done for the season. Last year I put out the 20 tubes and got 80. I figure 400-600% increase is pretty darn good. That doesn't include the ones I saw nesting in my shake shingle roof. I have Osmia Lignaria but next year I think I'll get some Californica. I may try the Humble Bumble Home too.
I've seen a lot more Honey Bees this last year since someone put out some hives about 1/2 mile from my home. Every little bit helps!
Happy gardening! Tom