The best time to take your houseplants outside depends on a few factors. These include the type of plant, your climate, and the current weather conditions. For example, a "rule of thumb" in my state of Kentucky is not to plant anything outside until after Derby Day. This is just when it's typically safe to avoid any overnight frosts. For your area, this might be different. Here are some general guidelines to follow.
Wait until the threat of frost has passed before taking your plants outside.
Because many houseplants natively grow in much warmer climates, they are very sensitive to cold temperatures and should not be exposed to frost. If you live in an area with cold winter climate, you should wait until the last frost date has passed before taking your plants outside.
Don't shock your plants.
Gradually acclimate your plants to the outside environment. Even though, eventually, many houseplants will thrive during their time outside, they need time to adjust. When you first take your plants outside, it's important to gradually expose them to the new environment. Start by placing them in a shady spot for a few hours a day. Over time, increase the amount of time they are in the sun (keeping in mind each plant's light preferences). This will help avoid leaf damage and give them the best chance of smoothly transitioning to their new home.
Check the weather.
Try to avoid exposing your houseplants to times of extreme temperatures (hot or cold) or excessively strong winds. These sudden changes can damage the leaves or send the plant into shock.
Consider the type of plant.
Some plants are more suited to outdoor conditions than others. Cacti and succulents will probably do better in the sunny, hot, dry conditions that would scorch the leaves of tropical plants used to growing in rainforests.
When to take your plants outside, an overview.
It might seem like a bit of a hassle to take your plants outside but it can be a great way to give them a boost of fresh air and sunlight (don't we all need that!). Just remember to do so at the right time and under the right conditions. What are you experiences transitioning your plants outdoors?