Tell them you were deathly sick.

"Yeah I had skin cancer. Nothing major but I had to take it easy and after beating it we all went on vacation to celebrate"

It may depend on where you live, but it may be illegal for an employer to ask about anything medical in an interview. I have a two plus year gap and if I get asked about it, I just say, "illness." They can't ask any follow up questions.

I have occasionally expounded on it, saying, "I'm fully recovered now." Just to reassure them that I don't expect to miss work due to illness in the future.