Definitely 5, 11 days would be the weekend after next. The only way it would be 11 is if the coming weekend had already been talked about, ie "I'm going to Pattaya this weekend and then Chiangmai next weekend."

Doesn't matter what the grammar books say, if you say "I'm going to Pattaya next weekend" on a Monday morning and you mean in 11 days then you will be misunderstood by 99% of the people you speak to.