help with to_date and to_char

Im trying to do something very simple
I have a field called when_month (integer )
so I want to get the month name for the integer

this comes close to what I want
update mytable set myfield=to_char(current_timestamp,'MON');

the result is myfield is set to OCT
which is close to what I want but when I try to use a variable
update mytable set myfield=to_char(when_month,'MON');
or a char with value of '05' or '5'
update mytable set myfield=to_char('05','MON');
get error:
update mytable set myfield=to_date(when_month,'MON');
update mytable set myfield=to_char('05','MON');
as well as
update mytable set full_month= to_date('01/'05'/2004','DDMONYYYY');
all get errors

Please can anyone help?