Reference for Pesachim 110:2
ת"ש ר' מאיר אומר כל מלאכה שהיא לצורך המועד גומרה בארבעה עשר ושאינה לצורך המועד אסור ועושין מלאכה בערבי פסחים עד חצות במקום שנהגו מקום שנהגו אין לא נהגו לא וש"מ לצורך המועד אין שלא לצורך המועד לא ש"מ:
[Thus,] only when required for the Festival, but not when it is not required! - No: the same law holds good that even when it is not required for the Festival we may also finish it, and he informs us this: that even when it is required for the Festival, we may only finish, but not begin. Come and hear: R'Meir said: Any work which is required for the Festival, he may finish it on the fourteenth; but that which is not required for the Festival is forbidden; and one may work on the eve of Passover until midday where it is customary [to work]. [Thus,] only where it is the custom, but if it is not the custom,it is not [permitted at all].