Hi,
You may pass on circular through email as below:
All employees are hereby advised to plan their leave well in advance with reason for leave and date of leave and get it approved by their respective Head of Department. Please avoid availing unplanned leave. In case of unplanned leave due to sickness or other reasons the same should be properly justified and leave should be informed to the Head of Department well before the commencement of your shift timing.
All are requested to adhere to the above very strictly.