HTML - Popup Win once per session

I have developed a website, and each month I have a pop up window that comes up within 3 seconds of entry on the main.html page. I would prefer that it only comes up one time when a visitor comes in. However, when they fill out the pop-up window andthen submits it, they will browse the site. Should they return to the main.html and stay there for more than 3 seconds, the pop-up window will come back up as per my code. I would like some advise on this. Thanks in advance!