Usually the middle to end of May is a good and safe time to move houseplants to the outside where they will benefit from additional light, airflow, and the occasional rainstorm. It is a time to move cautiously as it has been an unusually cool spring!
I have moved hardier plants like camellia, bay, fig, and rosemary outdoors. More tender plants like ferns, citrus and fuchsia should wait until evening temperatures remain above 45 degrees and the danger of frost has passed.
It is a good idea to move plants to the outside when the weather is expected to be cloudy for a few days, or to place them in the shade for three or four days to acclimate them to the outdoor light.
This is a good time to do any replanting of plants that may need a bigger pot, as it is much easier to do this on a tarp on the lawn than on the kitchen table! Houseplants outside will require more frequent watering, so I usually display them in groups for easier maintenance. A light topdressing of a time release general purpose fertilizer will give them all they need for vigorous summer growth.