Create a basic landing page, using our logo, with the message
"Closed until January 1st, 2018
We look forward to working with you in the New Year xx"
and put on the front page (hide the online store, so only this page shows)
I currently support 3 clients that run OSCommerce. I've done this exact thing several times before, with no issues. It's an extremely simple thing that's common when installing OSCommerce add-ons, making site alterations, or performing any other kind of site maintenance. The only part that will take more than a few minutes is making the HTML landing page, even that is negligible.