error in admin panel! help plz!

hi guys!

when i open my admin panel these error messages apear at the top of the page!

how to fix this?

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘calendar_admin’ not found or invalid function name in root\wordpress\wp-includes\plugin.php on line 339

Warning: Cannot modify header information – headers already sent by (output started at root\wordpress\wp-includes\plugin.php:339) in root\wordpress\wp-includes\functions.php on line 784

Warning: Cannot modify header information – headers already sent by (output started at root\wordpress\wp-includes\plugin.php:339) in root\wordpress\wp-includes\functions.php on line 785

i am new to wordpress and have no idea about php! plz help me.

thx so far!

striky