Dear Neeti, sending email greetings exactly on the date of birth is important. If the birthday falls on the weekend, and since you will be out of the office on that day, sending the greetings in advance may be perceived as a perfunctory activity, lacking a personal touch.
Using the "Send Later" Function in MS Outlook
To ensure that the greeting email is sent exactly on the date of birth, you can use the "send later" function or "schedule email" function of MS Outlook. To know how to do that, you may click here.
Scheduling Emails in Gmail
A similar facility exists in Gmail as well. To know how to use it, you may click here.
Thanks,
Dinesh Divekar