Any time of year is fine for buying or building and installing a bee house.  See the following website for more information.

• https://extension.umn.edu/lawns-and-landscapes/pollinator-nests

Thank you for the link to the pollinator nests article. It shows collecting hollow stems and bundling them into a container where bees can use them to nest. What I don't understand is when do you get rid of them? How do you know when the bees are through with them? Nothing I've read covers when it's safe to dispose of the straws or hollow reeds. More detail please. Thanks, Dianne

Here are two more articles.  It is best to leave them in place for two years.  This allows the bees to overwinter for two full years.  They will find another spot after that-perhaps another group of stems that you have left for them.
https://beelab.umn.edu/create-nesting-habitat
https://xerces.org/publications/fact-sheets/nesting-overwintering-habitat

Thanks so much. The xerces.org article gave much practical advice on managing my garden and landscaped areas to assist our little bees. -- Dianne

You are very welcome.  Thanks for looking out for the bees!