One should invest between 1st and 5th of every month in order to earn maximum interest from his PPF account.
The interest in PPF account is calculated every month and it is calculated on lowest balance in the account between 5th and last day of the month so if you deposit on/before the 5th of a month, you will earn interest for that month also.
For those subscribers who are interested to deposit lump sum amount, the ideal way to maximize the interest earning would be to invest Rs. 1.5 lakh (the maximum investible amount in a financial year) at one go on/before 5th of April every year.
For them who want to deposit some amount every month, should not forget to deposit the amount on/before 5th of each month to get maximum interest.