There are several ways to unset session variables in Magento. Most of these (not all) are defined in Varien_Object
and so are avialable to all objects in Magento that extend it.
unsetData:
$session->unsetData('day_filter');
$session->unsetData('days');
$session->unsetData('next_delivery_date');
uns (which will be marginally slower and ultimately executes unsetData anyway):
$session->unsDayFilter();
$session->unsDays();
$session->unsNextDeliveryDate();
getData
Not a mistake! A relatively unkown method exists in Mage_Core_Model_Session_Abstract_Varien
. The getData method in this class contains an optional boolean second parameter which if passed true will clear the variable while returning it.
So $session->getData('day_filter', true);
would return the session variable day_filter and also clear it from the session at the same time.
Set to null:
$session->setData('day_filter', NULL);
$session->setData('days', NULL);
$session->setData('next_delivery_date', NULL);
unsetAll | clear
Finally you could use the nuclear option (BEWARE: This will unset ALL DATA in the session, not just the data you have added):
$session->unsetAll();
or $session->clear();
(both aliases of each other)