In JEvents 2.0 you can customise the popup tooltips by editing them in the layout editing tool.

From JEvents 1.5.4 onwards it is easier to customise the tooltip/popups that appear in the monthly event window.

Assuming you are using the Joomla style tooltip then simply unzip and upload this file to templates/{YOUR TEMPLATE}/html/com_jevents/{YOUR LAYOUT}/month/ and customise the output (near the bottom of the file).  You'll aleaady see the event description and extra info fields there (commented out for now).

JEvents Component

Many component output options

JEvents Modules

Many customisable modules

Event Administration

Event Administration

Attend, Invite, Remind

Sell tickets, send invitations & reminders

Locations & Venues

Maps, addresses, venue information

YOOtheme Integration

Use JEvents data sources and features within YOOtheme.

Themes & Styles

Beautiful responsive themes

Custom Fields, Images, Tags & People

Extend your events

Social Engagement

Engage with your users

Who Sees What & When?

Decide who sees what, where and when!

Extra Modules

More module choices !

Link to content and CCKs

Link your events to articles