Yes , Its so simple and easy to send email to all contacts in Odoo.

Odoo has a functionality called mass mail,you need to do the needful as given below:-

Install mass mail module on your system.

Go to  Sales - Customer - Filter - Advance filter 

and search customer checkbox as true

Then save the filter as customer,

Now you can create a Template for Christmas in ( setting -  technical -email - Template) and assign object as res.partner.

Now go to mass mail module - start a compaign and click on new wave  and mail composer will open there select the model as res.partner and automatically in next field you will see the filter customer( which you have saved earlier) 

now you can select the Email template and click on send,

You will see the email sent to all customer and how many customer opens up your email. entire functionality is in built