The question of whether women still have to wear head coverings at Mass and, if not, how this can be documented, periodically comes up, so I thought I would deal with it here.
Under prior canon law, women were required to wear some form of head covering at Mass. Here is the relevant canon from the 1917 Code of Canon Law:
§1. It is desirable that, consistent with ancient discipline, women be separated from men in church.
§2. Men, in a church or outside a church, while they are assisting at sacred rites, shall be bare-headed, unless the approved mores of the people or peculiar circumstances of things determine otherwise; women, however, shall have a covered head and be modestly dressed, especially when they approach the table of the Lord.
The Code of Canon Law is a document that for the most part does not deal with liturgical law (see canon 2 of both the old and the new codes). As a result, whenever the Code does say something of a liturgical nature (like canon 1262), there tends to be an echo of it in the Church's liturgical books. This means that, when the liturgy was integrally reordered following Vatican II, the head covering requirement may have lapsed at that time since it was not repeated in the new liturgical documents. The promulgation of the new liturgical law may have overridden the liturgical provisions of the 1917 Code, just as many provisions of the Code were being overridden in the years leading up to the promulgation of the 1983 Code. While this is a possibility, I have not been able to verify it.
Nevertheless, it is certain that the legal obligation ceased with the release of the 1983 Code of Canon Law. The reason is that the new Code expressly abrogated the old Code, stating:
§1. When this Code takes force, the following are abrogated:
1° the Code of Canon Law promulgated in 1917;
The legal requirement made by canon 1262 of the 1917 Code thus lapsed with the abrogation of the 1917 Code itself. For the head covering rule to still be in force, it would have to have a different legal basis. However, the revised liturgical documents do not contain it, and neither does the 1983 Code. In fact, the new Code has no canon that parallels the old Code's canon 1262 (meaning that at Mass men and women no longer need to sit apart, men no longer need to remove their hats as a matter of law, and women no longer need to wear them).
Some recently have tried arguing a different legal basis for the head covering rule by appealing to custom. Canon law does provide for the possibility of customs obtaining force of law, but for this to happen several requirements must be met, as you can see from the following canons:
Can. 23 Only that custom introduced by a community of the faithful and approved by the legislator according to the norm of the following canons has the force of law.
Can. 25 No custom obtains the force of law unless it has been observed with the intention of introducing a law by a community capable at least of receiving law.
Can. 26 Unless the competent legislator has specifically approved it, a custom contrary to the canon law now in force or one beyond canonical law obtains the force of law only if it has been legitimately observed for thirty continuous and complete years. Only a centenary or immemorial custom, however, can prevail against a canonical law which contains a clause prohibiting future customs.
The argument that is made appears to be that the mandatory wearing of head coverings by women is an immemorial custom and thus obtains force of law per canon 26. The problem with this line of argument is that it involves a category mistake. Though we might colloquially speak of the "custom" of women wearing head coverings, this matter did not belong in the legal category of custom prior to its abrogation. It was not a matter of custom but a matter of law. The 1917 Code expressly dealt with the subject, so it was not a custom but a law that women wear head coverings in Church. That law was then abrogated.
One cannot appeal to the fact that, when a law was in force, people observed the law and say that this resulted in a custom that has force of law even after the law dealing with the matter is abrogated. If one could say this then it would be impossible to abrogate any long-standing law--or at least any long-standing law that people generally complied with--because mere law keeping would create a binding custom that would outlive the law.
This means that, following the abrogation of the head covering law, the faithful of the Latin church (the community supposedly still affected by the head covering rule) would have to introduce the practice as a matter of custom, intending it to gain force of law (per canon 25), following which the legislator of the Latin church (the pope) would either have to specifically approve the custom or it would have to be observed for a thirty year period.
Those things have not happened. The faithful of the Latin church did not introduce head coverings after the abrogation of the law regarding them. In fact, even when the subject was a matter of law, it was widely disregarded--so much so that the disregard is probably the reason the law was abrogated. The Latin faithful certainly did not introduce a head covering custom with the intent to bind themselves to observe it, so the requirement of canon 25 is not met. Further, the pope has not specifically approved this non-existant custom, nor has it been observed for a thirty years period, so the requirements of canon 26 are not met.
Also, canon 28 provides that: "Without prejudice to the prescript of can. 5, a contrary custom or law revokes a custom which is contrary to or beyond the law." Since the matter of women's head coverings at Mass is not dealt with in present canon or liturgical law, a custom involving it would be beyond the law and hence would be revoked by a contrary custom, which is what we in fact have had in the Latin church for the past thirty years.
The argument from custom thus does not provide a basis for a continuing legal obligation for women to wear head coverings at Mass.