We use the present continuous to talk about actions in progress now as well as for arrangements in the future. Many people know that 'will' and 'be going to' are used to talk about the future for plans and events but you can also use the present continuous. 'Be going to' tends to be used for plans while the present continuous is used for arrangements. The difference is that with an arrangement there is another person or money involved. In your sentence, the arrangement has been made between your cousin and your family. Hope this helps Bob

My cousin is spending his holidays with us next summer. ITS Wrong My cousin will spend holidays with us in next summer. CORRECT

"My cousin spends his holidays with us" denotes past, present and future actions. The proper form should be "Our cousin will be spending his holiday with us next summer".