This is absolutely correct. The reason behind this is that according to PPF Rules, the interest on PPF deposits is calculated on the minimum balance between the fifth of the month and at the end of the month.

Please note, interest on PPF deposits is calculated monthly but it is credited only at the end of the financial year. Therefore, if you are planning to make a lump-sum investment in your PPF account, financial planners recommend that you do it before April 5, in order to get the maximum amount of interest for your deposits.

This rationale holds true for monthly investments as well. Thus, you must deposit the money in your PPF account before fifth of every month.