I am writing this letter so as to give you the information which will help you organize all the necessary details of the forthcoming holiday.

forthcoming 是由动词 forthcome 的现在分词演变而来的形容词。forth 表示"向前"，come 表示"来"，合起来字面意思是"即将到来的"。在句中作定语修饰 holiday，指"即将到来的假期"。