You can try our Pop-up+ extension, which is designed exactly for the purpose you mentioned. The module allows you to specify start and end dates, popup durations, and you can customize it in any way you like.
Here is the product page and demo:
Product page - https://goo.gl/77iU1J
Demo Store - https://goo.gl/UT87AT
Popup for Magento helps you in creating attractive popups for your eCommerce store. Popups are great tool for grabbing website visitor’s attention and converting them into customers. This is a universal extension to create popups with any HTML content, image or form when pages load for grab customer attention. Promote your products, news, ads and events or collect information from users by any kind of form (newsletter or any custom form) through an attractive popup window.