And let them sign, with date, that they received your letter of resignation.
You don´t have a contract with the CEO, you have a contract with the company as legal entity. It is not your problem whom the company chooses to represent itself and who is on vacation. As soon as you letter of resignation is in the sphere of action of that company, they are responsible.
You just have to proof the date. You could also just send it as registered mail and it would suffice if the secretary signed it. But usually it is easiest to do it in person an just have a small paper-receipt with you for them to sign.