You should aim to buy your NSandI Premium Bonds near the end of the month. This is due to the fact that you must hold them for at least one calendar month before you are entered into a draw.
As each draw takes place around the 1st of the month if you buy around the 27th then you leave a couple of days for the transaction to go through, you sit out a full calendar month and are eligible for the quickest next possible draw date. This way you have the minimum waiting time before you are in a draw.
For withdrawals simply withdraw immediately after a draw is finalised to have your money back as soon as possible.