Example of how to add an events venues information into the calendars tooltips using the filter for the reg button HTML.

tw_ee_add_venue_info_before_reg_button.php

<?php //Do not include the opening PHP tag if you already have one.

add_filter( 
    'FHEE__EE_Calendar__get_calendar_events__tooltip_reg_btn_html', 
    'tw_ee_add_venue_info_before_reg_button', 
    10, 
    3 
);
function tw_ee_add_venue_info_before_reg_button( 
    $tooltip_reg_btn_html, 
    $event,
    $datetime
) {
    $venue_html = '';
    if($event instanceof EE_Event) {
        $venues = $event->venues();
        if(is_array($venues)) {
            $venue = reset($venues);
            if( $venue instanceof EE_Venue ){
                $venue_html = '<p class="time_cal_qtip">';
                $venue_html .= '<span>' . $venue->name() . '</span><br>';
                $venue_html .= '<span>' . espresso_venue_raw_address('multiline', $venue->ID(), false) . '</span>';
                $venue_html .= '</p>';
            }
        }
    }
    return $venue_html . $tooltip_reg_btn_html;
}