A complex sentence is a sentence consisting of an independent clause and a subordinating clause. Or, to put it more simply, complex sentences involve more than just a straightforward noun + verb. Complex sentences include extra phrases added to a base sentence, but which cannot stand alone as a sentence. For example:
Published 4:00 am, Thursday, January 9, 1997. 1997-01-09 04:00:00 PDT Yosemite National Park -- Yosemite Valley is slowly drying out after what park rangers are calling the worst flood here in recorded history, leaving behind hundreds of upturned tent-cabins and acres of wrecked camping sites.
Here's a climate chart: http://www.yosemite.ca.us/faq/1.html#climate. During high-snow years the Housekeeping Camp could flood in Spring (usually late May or early June), so you're probably safe with early May (although anything's possible, especially if a (unusual) warm, tropical rain occurs).