A sysadmins job on a monthly and daily basis can vary but A sysadmin will usually be performing tasks such as installing software, server programs, updating the server, installing and updating scripts, patching and securing the server, monitoring the up time of the server and making sure everything is working correctly on the server.
Sometimes a sysadmin will perform some server programming and will work to bring a server back online should there be an outage of the server.
A sysadmin will also check server logs and update apache, mysql, php and ensure complete proper operation of things installed on the web server and the server itself.