A lot is said about supposed Vatican or papal documents on feminine modesty in dress. But the Internet is not exactly great on providing exact information. So here I present a literal (but unofficial) translation of an actual Vatican document.
(The bad news is that the OCR is terrible, and the book is not in public domain in the US. I will try to find an original volume or some microfilm, if I can remember to do it.)
You can find the Latin document in Acta Apostolicae Sedis, Vol. 22 (1930), on pages 26-28.
Here is that volume on vatican.va.
Here is that volume on Documenta Catholica.
Instruction to Diocesan Ordinaries:
“De inhonesto feminarum vestiendi more”
(aka “On a degrading custom of female dress”)
Issued by: The Congregation of the Council (Sacra Congregatio Concilii — now subsumed into the Congregation of the Clergy)
Unofficial translation: Maureen S. O’Brien
Our Most Holy Lord Pope, Pius XI, with the force of his supreme apostolate, which is performed within the whole Church, never ceased to teach what St. Paul did — that is, “The women in decorative* apparel, adorning themselves with modesty and sobriety… and with good works, as is right for women professing piety.” (1 Tim. 2:9-10)
And numerous times as occasion was given, the same Supreme Pontifex expressed disapproval sharply, and condemned a degrading custom of dressing among both Catholic women and girls, introduced today here and there, which not only offends gravely against feminine splendor and ornament, but even more is also the true ruin of these women in the temporal world — and what is worse, throws others into the most miserable everlasting ruin.
Therefore, it is nothing strange if bishops, and others in the office of Ordinary, have also opposed this kind of crooked license, and every kind of forwardness, in their own dioceses and with one voice — enduring, with a strong and patient soul, no few mockeries and scorn for this reason, brought upon them by people of bad will.
And so, let this Sacred Council describe the vigilance and merited action of this kind of Sacred Prelate with approval and praise, to the clergy and people of approved discipline. Then may it encourage them vehemently, so that they may persevere what they have undertaken, with advice and a suitable beginning. And let them urge eagerly for the sake of men, until this deadly plague may be rooted out from the honest association of humans.
So that it may be put into effect more easily and safely, this Sacred Congregation has decided to establish what follows:
I. As occasion is given, let parish priests and preachers particularly “insist… reprove, entreat, rebuke” (2 Tim. 4:2) about how women wear clothes, so that they may understand modesty, that they may be the ornament and guard of virtue; and let them warn parents not to allow their daughters to wear disgraceful clothes.
II. Mindful of that most grave obligation which they hold, to care for the morals of their offspring and for their first religious education, let parents show particular diligence so that their daughters may be set up solidly in Christian teaching, from their first years; and let them eagerly kindle the love of the virtue of modesty and chastity in their souls, by word and example. Let them be busy with imitating the Holy Family’s example in their own families, in order to set up and govern their families in that way. And so let each family have a cause and an invitation for loving and serving modesty behind its domestic walls.
III. Let parents keep their daughters away from public drills and assemblies of exercise [in immodest clothes]. If their daughters should be forced to take part, let them take care that their clothing shows that they have dignity. Never allow them to wear degrading clothing.
IV. Let principals of colleges and schoolteachers strive to embue the souls of girls with a love of modesty, so that they may be efficaciously influenced to dress worthily.
V. Let these principals and schoolteachers not admit girls that wear clothes that are less than worthy, into colleges or schools; and let those who have been admitted be sent back to their mothers, unless they stop and become reasonable.
VI. Let religious women** not admit girls, nor tolerate those already admitted, who will not keep a Christian custom of dress, in their colleges, schools, oratories, or recreation facilities, according to the letters from August 23, 1928 sent by the Sacred Congregation of Religious. Indeed, let them show particular care in educating female students, so that the love of holy bashfulness and Christian modesty may drive deep roots into their souls.
VII. Let pious women institute and encourage associations of women, which by advice, example and goal of work may decide ahead of time to restrain abuses in the wearing of clothes, by the Christian modesty which disagrees with it; and to promote purity of customs and worthiness of dress.
VIII. Let no one who wears degrading clothing be admitted into pious associations of women. Indeed, if an admitted woman should go wrong in such things afterward, and she will not act reasonably after being warned, let her be expelled.
IX. Let girls and women who wear degrading clothing stay away from Holy Communion, and from the office of godmother for the Sacraments of Baptism and Confirmation. And if the case warrants it, let them be prohibited from entrance into their churches.
X. When festivals occur throughout the year, let parish priests and sacerdotes, as well as heads of pious Unions and Catholic guilds, take the particular opportunity to teach Christian modesty to women, especially on feasts of the Blessed Virgin Mary. Do not let the opportunity pass to call them back and excite them for dressing according to Christian custom. And on feasts of the Blessed Virgin Mary conceived without sin, let them complete the particular prayers in every cathedral and parish of the Church, whatever the year; where one can do so, let it be considered an opportunity for encouragement, during solemn sermons for the people.
XI. Let the diocesan council act with vigilance upon what was given in the declaration of the Holy Office on March 30, 1928, advising women efficaciously on better means and reasons for modesty, at least once a year, by declaration.
XII. Indeed, so that this healthy action may advance more efficaciously and safely, it is preferred that bishops and others in the office of Ordinary, also in the third year, alone and with relation to religious institutions, should observe the norms of that instruction given in the Letter “Orbem Catholicum” from June 29, 1923, also upon the condition and status of things concerning the custom and works of women’s dress.
Given at Rome, from the chair of the Sacred Congregation of the Council, on January 12, on the Feast of the Holy Family, in the year 1930.
Donato Cardinal Sbaretti Tazza, Bishop of Sabina-Poggio Mirteto, Prefect.
Giulio Serafini, Bishop of Lampsacus, Secretary.
* Greek: kosmein – “decorated”, with connotations of “orderly, decorous.”
Latin: ornato – “decorated, ornate.”
** religious women = nuns and sisters.
Stuff to notice:
1. Nothing is said about the particular fashions being condemned.
2. No particular hemlines, necklines, or other measurements are mentioned.
3. Totally okay with Italian churches making you wear more clothes, if you want to see the indoors.
4. That said, it’s also totally okay for laywomen to start clubs and apostolates about their concerns.
Other stuff to notice:
This site has a totally different wording for this decree. Their Latin decree appears to be on a similar topic but written in 1954, from the same Congregation but with different folks in charge. Obviously I need to check the 1954 volume of Acta.