İyi tatiller! Umarım tatilin iyi geçer. Umarım iyi bir tatil geçirirsin. All of them are same. You can say "İyi tatiller!" to a group of people and to just one person. And the other options are for telling to just one person. If you change the suffixes and say Umarım tatiliniz iyi geçer. Umarım iyi bir tatil geçirirsiniz. Then you can tell this a group of people (or a person who is not close to you). İyi tatiller is the most common option and you can say the others for expressing your good wishes for your close friends. And another option is "İyi tatiller dilerim!" this is a short way for telling "Umarım iyi bir tatil geçirirsin(iz)."

Thanks sis ..

All is right! Same meaning with different centences... You can use what you want... =)