This is from org mode forum:

I am reading the Org-mode Release 6.27a manual to learn new things.
and just discovered the C-c C-x c (clone subtree) command.

This makes copies of a tree and inserts them as siblings. The command
prompts you for the number of copies to make. The really useful
feature is to update dates according to a pattern.

Here is an example of creating headings for each of the next six months:

Start with this:

** First of the month
*** July
SCHEDULED: <2009-07-01 Wed>

Position the cursor to the beginning of the line *** July
then issue the C-c C-x c

Answer the question “Number of clones to produce:” with 6

Next question is
“Date shift per clone (e.g. +1w, empty to copy unchanged): ”

Answer it +1m and the tree looks like:

*** July
SCHEDULED: <2009-07-01 Wed>
*** July
SCHEDULED: <2009-08-01 Sat>
*** July
SCHEDULED: <2009-09-01 Tue>
*** July
SCHEDULED: <2009-10-01 Thu>
*** July
SCHEDULED: <2009-11-01 Sun>
*** July
SCHEDULED: <2009-12-01 Tue>
*** July
SCHEDULED: <2010-01-01 Fri>

Now you just have to change the occurrences of July to the real
month names!