Calm down Mis-student...
On a day
at a time
in everything else - moth, year, century, etc.
"On Monday" "On the 27th"
"At seven o'clock"
"In 2007" "In September"
'will' is one of several ways of talking about the future, and is used for a future intention. It has no connection with 'be having', unless the future intention is to have (posses) something.
"I will cook dinner"
"I will go to the shop"