This is an article I thought was already on the blog, but couldnt find it. So, here goes. First lets put this into perspective. I live in Southern Ontario - Zone 6B - near Niagara Falls. If you live in Sudbury or Winnipeg or Peace River Alberta you're not as warm as it is here.
If you're on the Prairies your summer air might not be as humid as mine.
Keep this stuff in mind and alter any directions to suit your locale.
Welcome to spring. The birds are singing, the sun is higher in the sky making the light more direct and stronger and just maybe it's stopped raining.
Let's talk about moving your bonsai trees outside for the warm weather.
There isn't a single tree that will fail to benefit from a summer outside. They get better light, better air movement, better humidity and the wonderful summer will play a big role in helping your trees make it though the winter inside. Remember, the only reason that bonsai trees are indoors for the winter is because they originate in tropical or sub-tropical climates and they won't survive our freezing winters. 
Any tree that can survive the winter- must winter outside-so if you have acquired a pine or maple or cherry bonsai and you bring it indoors, you will kill it.
How to move a tree outside without burning them.
The spring sun is strong and if you move a tree outside and immediately put it in direct sun, you're going to burn it. Start by placing your trees outside in the shade which is probabaly still brighter than it is in the house. After a few days give them about an hour of morning sun- the earlier the better. Watch your trees for any sign that they are burning. Gradully increase the amount a light they get until ( for most trees) they are getting full sun up until about 1:00 pm and then getting shade in the afternoon. Surprisingly it seems that the time to really stay away from is the late afternoon. Don't take my times as Gospel- pay attention to your trees and see how they are tolerating the outside.
Two major factors in the summer affect how much water your trees need. The first is their rate of growth and the second is the weather. Your trees are going to put on a major growth spurt and that means they need more water. For some trees ( junipers for example) when the weather gets really hot they will actually slow down their growth, but they are still going to need lots of water because it's so darned hot. Remember your bonsai trees are not from a temperate climate like the maples and pines, they are tropical so bring on that sun and that heat- they love it.
You might need to water them every day. If they are planted in real bonsai grit you might have to water them twice a day (another reason that Zen Garden does not use bonsai grit).
Dealing with the rain. I talk about needing to water your trees, but of course if its raining for days on end, the last thing they need is more water from you. For the summer- put the drainage trays away if you can. You dont need them. If your trees are getting too much water you might need to move them to a more sheltered spot, but first you can try wedging one side of the pot higher which will send the water to the low side of the pot and encourage better training. Whatever you do make sure they are not sitting in water.
A special note for Chinese elms. When they put out new foliage and for as long as it's still that lovely bright green and the leaves are still quite soft (new leaves look differnet from old leaves) try and keep them out of the rain. Water on new leaves can encourage a fungal infection that looks like black spots.
Finally, Feed your tree over the summer for sure.