This mod will popup message on every page load or once per session between selected dates or on selected weekdays. Message is predefined with smarty template so you can insert whatever you want in it and formatted any way you want.
You can also show short messsage at the top of center part of the site again on every page load or once per session. The short message also allows html tags.
You can get the mod here -
http://www.cflsystems.com/holidays-events-message-mod-for-x-cart.html
The mod work on 4.1.x, 4.2.x, 4.3.x and 4.4.x
http://www.cflsystems.com/public/everyone/holiday_msg_admin_settings.png
http://www.cflsystems.com/public/everyone/holiday_msg_popup_message.png