You can also set DirectoryIndex in apache's httpd.conf file.
CentOS keeps this file in /etc/httpd/conf/httpd.conf
Debian: /etc/apache2/apache2.conf
Open the file in your text editor and find the line starting with DirectoryIndex
To load landing.html as a default (but index.html if that's not found) change this line to read:
DirectoryIndex landing.html index.html