Dear Pratap,
This is in addition to what our senior member KK!HR has said. While he has recommended that junior members should come up with the draft of the mail, I would like to go little further andlike to raise question on the efficacy of the policy itself.
Birthday celebration is personal matter of the employee. This kind of policy could promote devotion to the pleasure or self-gratification. Celebrating birthday of an employee at the workplace is one thing and giving employee half-day off is another. Birthdays are celebrated at the workplace in order to promote the workplace interaction, inject element of fun etc. Giving half day off on account of birthday is completely different matter.
Possibly today your company could be financially strong and this has made you to take this type of decision. Nevertheless, there are ups and downs in the life of the company and in future you might be forced to withdraw this facility. In such case, it could cause heartburn to the employees. Secondly, due to work pressure, what if particular HOD is unable to spare junior employee for half day off?
In view of the points mentioned, I recommend you reevaluating this policy. Nothing wrong in the introduction of the welfare measure for the employees, however, these should be common and not person specific.
Thanks,
Dinesh Divekar