Edit your header Layout under the Appearance > Layouts menu
Add a content block widget to the bottom of your layout. Stretch the widget full width. Click the cog icon to open the widget to edit it. Add your message to the content area and user the WYSIWYG tools to style the message. Click publish.
After the holiday to remove the message, simply add the CSS class "hide" in the CSS Class field and it will hide the message from the storefront. .
Note: If your site has a custom design that includes a sticky header, please reach out to Customer Support for assistance with implementing the message.