how to schedule to display web page maintenance every month ?

Whilst it's possible, iRules are event driven so it's not very practical to schedule them to display something at a certain time. It might be easier to automate applying a different iRule to the virtual server at the times you want to display the maintenance page

Hi PC,

You can trig your maitenance page using Irule above:

when HTTP_REQUEST {
   "clock seconds" will return the time in seconds
   "clock format" will format the previous value with it's date/time components
   split will chop the returned string into a list of elements.
  set l [split [clock format [clock seconds] -format {%A %H}] " "]

   Next add the time check logic:
   This example will process the log statement on Sunday between 3 and 4am.
  if { ([lindex $l 0] eq "Sunday") &&
       ([lindex $l 1] >= 3) &&
       ([lindex $l 1] < 4)  } {
     Put in iRule logic here
    log local0. "processing iRule..."
  }
}

For info i retrieve the irule in this post:

https://devcentral.f5.com/questions/schedule-an-irule-for-specific-date-time