We inquire about and expound upon the laws of Pesaḥ beginning thirty days before Pesaḥ. We learn this from Moshe, who on Pesaḥ itself explained the matter of Pesaḥ Sheni, the make-up date for those unable to bring the Paschal offering, which takes place thirty days later. The main reason for this is that all of Israel had to prepare animal sacrifices as Pesaḥ approached, examining them to be certain that they were free of disqualifying blemishes (Pesaḥim 6a; Avoda Zara 5b).
This enactment was not canceled even after the Temple was destroyed; it is proper to study the laws of Pesaḥ thirty days before the holiday arrives. As is well known, Pesaḥ has very many laws, pertaining to preparing the home for Pesaḥ, seeking and destroying ḥametz, baking the matza, and the Seder. Some Rishonim maintain that the enactment applies specifically to Torah scholars, enjoining them to prioritize answering practical questions about the upcoming holiday. According to this view, there is no universal obligation to set a fixed time for studying the laws of Pesaḥ (Ran and Rashba). Nevertheless, since many Rishonim maintain that it is indeed obligatory to set a fixed time for studying the laws of Pesaḥ beginning thirty days before Pesaḥ, it is proper that every individual do so, beginning on the 14th of Adar (Purim). It is also proper for schools and yeshivot to set a fixed time for studying the laws of Pesaḥ during this period.
There is a dispute amongst halakhic authorities about whether one is obliged to study the laws of the other holidays thirty days in advance. Some say that this enactment was established primarily for preparing the animal sacrifices, and such sacrifices were in fact brought on the three pilgrimage festivals – the olat re’iyah (pilgrimage burnt-offering), shalmei ḥagiga (pilgrimage peace offerings), and shalmei simḥa (festival peace offerings) – and it is therefore proper to study the laws of each festival thirty days in advance. Others say that the practice today primarily concerns Pesaḥ, since its laws are so numerous and strict (MB 429:1).
These differences of opinion and distinctions concern advance preparations for the holidays. On the holidays themselves, however, there is an ancient enactment of our teacher Moshe to study the laws and spiritual meanings of that holiday (Megilla 32a, MA 429:1).
It is also worth noting that some authorities maintain that the main obligation is for rabbis and Torah teachers to begin teaching the laws of Pesaḥ thirty days before the festival, but there is no obligation on each individual. This is what Ḥok Yaakov states in 429:1, 3, adding in the name of Roke’aḥ, Raavan, and Kol Bo that even the reading of Parashat Para right after Purim was established to remind the people to purify themselves for the upcoming Pesaḥ. Similarly, many Aḥaronim write that this is the reason for the establishment of the custom to teach the laws of Pesaḥ on Shabbat Ha-gadol, as recorded in SAH and MB 429:2. Nevertheless, according to most authorities there is still a mitzva for every individual to delve into the laws of Pesaḥ during the thirty days prior to the festival. BHL rules accordingly. However, there is arguably a greater obligation for rabbis and teachers.