how long does it take for an egg to fertilize after unprotected sex??

EJane • Boy Mom x2
Hi everyone!
So I'm looking to conceive in August, so that hopefully I would have a May baby... when should I start trying, late July? Or August... let's say once I have unprotected sex would a sperm reach an egg and fertilize right away? 
I'm a bit confused about the process... according to my calendar, I'm going to get my PD last week of July and ovulate mid-late August...