Thirty days before Pesah, one should inquire about the halakhoth (laws) of Pesah. We learn this from the fact that Moshe Rabbenu, 'a"h, explained the laws pertaining to Pesah Sheni (which takes place one month after Passover) on Pesah (Passover) itself.
From this, there are those who learn that the laws of all Holidays must be learnt a month before the Holiday, whereas others say that this only applies to Pesah. Pesah has an additional reason as to why one must study one month in advance, and that is because its laws are complex and numerous.
Even though the Gemara (Talmud) also mentions that one must give Derashoth (homilies) from a month before Pesah, the Shulhan 'Arukh (Code of Jewish Law) does not mention this aspect. The custom nowadays is that the Derashoth are given on Shabbath Haggadol (usually the Shabbath before Pesah) when the Rabbis of the communities expound on the Holiday and then give Halakhoth (laws) pertaining to Pesah.
(See Shulhan 'Arukh 429: 1. Mishna Berura. ibid 1,2. Kaf Hahaim ibid, 1,2)
