Musar for Makkot 45:4

And the content of one who spurns the festivals, is that he does work on the intermediate days of the festival and spurns the punishment, because the prohibition of work on the intermediate days of the festival is not explicit in the Torah. And this is in the way of one who is a heretic for one thing, to anger - as we have explained, such that he has no share in the world to come. And our Rabbis, may their memory be blessed, said (Bekhorot 30b) [that] one who comes to convert and says, “Behold, I accept all of the Torah except for one matter from the words of the Sages” - we do not accept him. And they, may their memory be blessed, said (Makkot 23a), “One who spurns the holidays is like one who worships idolatry, as it is stated (Exodus 34:18), ‘You shall observe the Feast of Unleavened Bread’; and adjacent to it is ‘You shall not make molten gods for yourselves’ (Exodus 34:17).” And they, may their memory be blessed, explained (Chagigah 18a), “‘You shall observe the Feast of Unleavened Bread’ - observe it to not do work all of the Feast of Unleavened Bread.” And we were warned with this about work on the intermediate days of the festival. And that which it is written (Leviticus 23:39), “a complete rest (shabbaton) on the first day, and a complete rest on the eighth day” is because there are many types of work that are permitted on the intermediate days of the festival, as is explained in their words, may they be blessed.