Certified GKH Gardening Expert
Lemons and other citrus plants actually bloom when they are under mild stress, so typically blooming occurs during the cooler months. That being said, it is not uncommon to also see a citrus tree bloom during drought or when affected by disease or pest.
When a lemon tree is in bloom, it is a good idea to fertilize lightly and increase water to the plant. If your tree is isolated (i.e. does not have access to pollinators) you will also want to use a paintbrush to gently "paint" the inside of some (but not all) of the blossoms. This will help to ensure fruit. You do not want to pollinate all of the blossoms because a tree will produce far more blossoms than it can support the fruit of and you will get better fruit if only a few of the blossoms produce fruit.