It is always good to start your holiday letter by conveying your warm greetings and even acknowledging the current occasion, festival or holiday. After that just proceed to the main event of your family which could be anything like announcements, achievements, funny incidents or others.
In this part of the letter in order to show your concern towards the addressee enquire about them and if possible also mention some of your shared memories if any. Holiday letters are just perfect for mentioning your old good memories of holidays spent together. If you wish to send some money or a picture of your family as a token of love then do mention it in your letter.
In this last part of the letter you end by conveying your good wishes to the addressee and his family and also mention your address and other contact details.