I volunteer at our zoo, so the pay for me is nothing. But as far as our keepers go, we have the zoo divided into 4 areas (its a small zoo) so there are 4 full-time keepers for each area, 2 relief keepers that work full-time split up between each area covering the main keeper's days off, and one part-time keeper. (One of our full time has the title of "head keeper"
. They do have set days off, but no one gets a full weekend (in other words someone gets Sun-Mon off and another gets Fri-Sat). I'm not sure on pay, but it is fairly low since we are a city run zoo. Privately or county funded zoos may be able to pay more, really depends. Our zoo is only closed to the public on Thanksgiving, Christmas, and New Years day, so on those 3 days the keepers can put in a "short" day, only getting done what absolutely needs to be, which is still a good 5 hour day. Otherwise they work 8-5, taking lunch from 1230-130. Its definately a job you do out of love, and not for money. If you have any other questions, let me know, despite just being a volunteer, I have spent the last 2 years working with the keepers directly and have so much time and experience that they actually called me in when a keeper called in sick one day and no one else could come in. How sad is that?? 
Kati