What makes big dead spots in my Lawn

Could be a number of reasons,if you summit a photo,it will be easier to answer your question

If it's happening July/August it could be grubs. They become active during that time.

I've read that treating the dead spots with epsom salt will help them recover and turn green again, as to what is causing them, I'm not 100% sure, do you or your neighbors have animals?

the first suspect would be grubs. They are destructive little buggers but very easy to kill with a lawn insectide/pesticide available at any garden supplier, Lowes, or Home Depot. It works fast. However you will need to rake out the dead spots and reseed.

Your dog! (well, it's possible!)