"However, if a person is in the habit of observing Saum on a particular day" means that if it is his routine to observe Saum on these days, then he can do so two days before the advent of Ramadan. The reason being that in that case his Saum will not be for the reception of Ramadan but a matter of routine. Some people have taken "one or two days before" to mean the first two days of the second half of Shawwal on the grounds that some Ahadith contain prohibition of observing Saum in the second fortnight of Shawwal. Because of this injunction, it is not permissible to observe Saum in these days, except in case it is one's routine.