There are several sources of templates online, many of them offering free templates.
Most are either PDF or MS Word format.
PDF files are usually images, which you can insert into a Pages document, expand to fit the page, then move to the Section Master where it will be behind the text you add to the page. You can add the text to the Document Body of a word processing document (you'll need to set wider top and bottom margins to keep the text from running over the border decorations), or you can create a Page Layout document by removing the Document Body (Document inspector, Document tab, uncheck Document body checkbox). then adding one or more text boxes to hold the text.
Pages can open MS Word files, but will give you a warning message and may lose some elements not supported in he same manner in Pages as in Word:
Despite the warning, I saw no large difference between the Pages document and the image of the template on the website. The font change applied only to text in the address block at the top of the page (where you'd replace it in the text box with your own address, an choose your own font) and to the "start text here" note you'd replace with the start of your letter.
I used DuckDuckGo to search for 'holiday letter template'