【答案】They are going to hold a big birthday party next week.

【解析】

下周next week；他们they；将，用一般将来时be going to do ，主语是they，be动词用are，变成are going to；举行hold；一个a；大型的big；生日birthday；聚会party。该句是陈述句，时间往往放句子末尾，故答案为They are going to hold a big birthday party next week.