When the Temple stood, one who offered a sacrifice was prohibited from performing melakha (doing productive work) that day, as it was like a holiday for him, and it is inappropriate to perform melakha while one’s sacrifice is being offered up on the altar. The same applies to Erev Pesaĥ, a day on which Israel is obligated to offer the korban Pesaĥ (Paschal sacrifice), and which is consequently like a holiday for everyone. But since the time for offering the korban Pesaĥ does not begin until midday, the prohibition against melakha also does not begin until midday. In some places, people adopted the stringent practice of refraining from melakha from the morning of the fourteenth of Nisan, and in such places this custom was binding. Even now, after the destruction of the Temple, this prohibition remains in effect, and it is forbidden do perform significant melakha after midday on the fourteenth of Nisan. This custom also prevents one from becoming preoccupied with work and forgetting to destroy his ĥametz and prepare matza, wine, and other necessities for the Seder night.
Indeed, the Sages instituted a ban on significant melakha every Erev Shabbat so that people would be free to prepare for Shabbat. However, the prohibition on Erev Pesaĥ is more severe, for while the Sages taught that whoever performs melakha on Erev Shabbat will see no sign of blessing from it, those who performed melakha on Erev Pesaĥ were actually excommunicated.
In practice, the halakha is that melakha is forbidden on Erev Shabbat from the time of minĥa ketana (two and a half seasonal hours before sunset), while the prohibition on Erev Pesaĥ begins at midday.
. The “korban” reason is explained in y. Pesaĥim 4:1 and is the opinion of Tosafot, Rambam, and most poskim. Tosafot implies that this prohibition is biblical in nature, but according to Rambam and many other poskim, the prohibition is rabbinic. All agree that after the destruction of the Temple the prohibition is only rabbinic. The first reason is the primary one; therefore, if the first night of Pesaĥ coincides with Motza’ei Shabbat, one may work until minĥa ketana on Friday just like on a regular week, even though he burns the ĥametz and takes care of other Pesaĥ preparations on Friday (BHL 468:1; see also Ĥazon Ovadia vol. 2 p. 82 and Hilkhot Ĥag Be-ĥag 14:8). The prohibition against work on Friday afternoon is explained in Pesaĥim 50b, and the Rishonim are divided as to whether the prohibition refers to minĥa gedola (5.5 seasonal hours before sunset) or minĥa ketana (2.5 seasonal hours before sunset). Because this constitutes a doubt about a rabbinic law, halakha follows the lenient opinion (see SA 251:1 and MB ad loc.; Peninei Halakha: Shabbat 2:8).
See MB 468:12 and Kaf Ha-ĥayim 32 ad loc. regarding the modern day custom regarding Erev Pesaĥ before midday. The fact that they wonder about the custom in Jerusalem makes it clear that there is no fixed custom to prohibit work, since where work is prohibited, it is also prohibited for guests, and certainly for the residents of the community. There are no communities nowadays that prohibit work on Erev Pesaĥ before midday.
However, according to SA 468:5 in the name of Rambam, even where there is no custom to prohibit work, only three types of craftsmen are permitted to begin working before midday: tailors, barbers, and launderers, since their work is done expressly for the holiday. For everyone else, if one began a task before the morning of the fourteenth and the work is necessary for Pesaĥ, he may work until midday, but if he did not begin before the morning of the fourteenth, he may not start. According to Rema, based on the opinion of most Rishonim, including Rashi, Raavad, Rosh, Ran, and others, the aforementioned distinction only applies where the custom is to refrain from work (like in Rema’s community), but where the custom is to permit work until midday (like in our communities), everything is permissible. This is the custom of the Ashkenazic communities. Even some Sephardic communities follow Rema; see Kaf Ha-ĥayim ad loc. 32. In any event, we follow the lenient view when there is uncertainty regarding a rabbinic law.