The cavity-nesting species that live in our bee homes are active any time it is warm and there is a source of food. For most places, that's from late March until early September, though it may start earlier and run later for you if you live in a warmer location. If you've heard otherwise, that's likely because some other insect hotels are only designed for a single species of bee, which means they will only attract a small subset of species that are active early or late in the year.