Enable Includes module
sudo a2enmod include
Editing config
Open the /sites-available/default file
sudo vim /etc/apache2/sites-available/default
Find section <Directory /var/www/> and edit it to:
<Directory /var/www/> Options Indexes FollowSymLinks MultiViews +Includes AllowOverride None Order allow,deny allow from all AddType text/html .shtml AddOutputFilter INCLUDES .shtml </Directory>
Restart Apache
sudo service apache2 restart
Test it works
Create a SSI test file /var/www/http/ssi.html
<html> <head> <title>SSI Test Page</title> </head> <body> <!--#echo var="DATE_LOCAL" --> </body> </html>