Rashi writes that the placement of the parsha of Shabbos before the parsha commanding the building the Mishkan teaches that Shabbos is more important than the Mishkan; from here we learn that one is not permitted to violate Shabbos even for the noble task of building a Mishkan. If so, since the Torah places the mitzvah of kibud av before the mitzvah of Shabbos, "Ish imo v'aviv tira'u v'es Shabsosai tishmoru," it should follow that kibud av is more important than Shabbos -- one should be allowed to violate Shabbos to do a parent's bidding. Why does the halacha not entertain such an idea?
Those learning daf yomi will remember that items which are tadir and items which are mekudash take precedence over items which are not. The gemara in Zevacim debates what to do when one has a tadir item on one hand and a mekudash item on the other -- which comes first -- and comes to no clear conclusion. The Rambam paskens one can take either one first.
Chasam Sofer explains: the mitzvah of kibud av is tadir because it can be done all week. Therefore, it takes precedence in the pasuk over Shabbos, which comes only once a week. Shabbos is tadir relative to the Mishkan -- Shabbos comes every week; there is only one opportunity to build a Mishkan. The only possible justification for placing the parsha of the Mishkan before the parsha of Shabbos would be if the Mishkan was more mekudash than Shabbos, in which case its construction should be permitted even on Shabbos. Since the Torah places Shabbos first, QED that we cannot conclude that the Mikdash is more mekudash, and we have no source to permit its being built on Shabbos.
I told this C.S. to a daf shiur expecting to be jumped on with questions, but no one took the bait, which leads to to suspect I'm missing something. The gemara in Zevachim draws a distinction between tadir and matzuy. Tadir means there is an obligation to frequently or repeatedly do a mitzvah. Matzuy means something happens to come up often, but there is no chiyuv forcing it to recurr. Korbanos shelamim may be offererd far more frequently in the mikdash than korbanos chatas, but they are matzuy, not tadir -- shelamim are usually donated voluntarily; there is no chiyuv compelling one to offer them. A person can eat multiple times a day, but bentching is not called tadir because one can choose not to eat at all (Sha'agas Arye #21). The Sha'agas Arye goes so far (#28) as to say the mitzvah of tzitzis is not called tadir because the mitzvah is conditional on wearing a four cornered garment and there is no chiyuv to don such a garment. Even if one chooses to do so, that does not change the nature of the mitzvah.
Is the mitzvah of kibud av tadir or matzuy? I think it belongs in the same category as tzitzis, matzuy. So long as a parent has no requests, no action is required to fulfill kibud av. True, there are things one should not do because of yirah for parents, but when we discuss whether kibud av is doche Shabbos or not, we are speaking of aspects of the mitzvah that require action. I don't understand why the Chasam Sofer classifies kibud as tadir -- do you?