First you need to open your Gmail account and select the Christmas email that you want to send. When the email is ready you click the 'forward' button below the email and then at the top of the 'forward' page you click 'to' (this option is next to the addresses list).

Under 'search my contacts' you wil find a 'my contacts' option. This will reveal your list of contacts and you can then click 'all contacts' and then 'done'. Finally, click 'send' and this should forward the email to all your contacts.

You can find more detailed instructions here, and on the right of the same page you can also find advice on how to select specific contacts, for example if you want to send a particular greeting to only certain people. If you have any further questions, the Gmail account page has plenty of helpful links.