Mourning is required during the `Iddah of Death
[فَإِذَا بَلَغْنَ أَجَلَهُنَّ فَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا فَعَلْنَ فِى أَنفُسِهِنَّ بِالْمَعْرُوفِ وَاللَّهُ بِمَا تَعْمَلُونَ خَبِيرٌ]
(...then when they have fulfilled their term, there is no sin on you if they (the wives) dispose of themselves in a (just and) honorable manner (i.e., they can marry). And Allah is well-acquainted with what you do.)
This Ayah indicates that mourning for the dead husband is required until the `Iddah is finished. It is also reported in the Two Sahihs that Umm Habibah and Zaynab bint Jahsh narrated that Allah's Messenger said:
«لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ وَالْيَوْم الآخِر أن تُحِدَّ عَلى مَيِتٍ فَوْقَ ثَلَاثٍ، إِلَّا عَلى زَوْجٍ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا»
(It is not lawful for a woman who believes in Allah and the Last Day to mourn for more than three days for any dead person except her husband, for whom she mourns for four months and ten days.)
It is reported in the Two Sahihs that Umm Salamah said that a woman said, "O Messenger of Allah! My daughter's husband died and she is complaining about her eye, should we administer kohl in her eye'' He said, "No,'' several times upon repeating this question. He then said:
«إنَّمَا هِيَ أَرْبَعَةُ أَشْهُرٍ وَعَشْرٌ، وَقَدْ كَانَتْ إِحْدَاكُنَّ فِي الْجَاهِلِيَّةِ تَمْكُثُ سَنَة»
(It is four months and ten (nights)! During the Jahiliyyah, one of you would mourn for an entire year.)
Zaiynab the daughter of Umm Salamah said (about the pre-Islamic era of ignorance), "When the woman's husband died, she would go into seclusion and would wear the worst clothes she has. She would refrain from wearing perfume or any adornments until a year passed. She would then come out of seclusion and would be given dung that she would throw. Then an animal would be brought out, a donkey, a sheep, or a bird. Then some blood would be drained from it, usually resulting in its death.''
In short, the mourning required from a wife whose husband dies, includes not using beautification aids, such as wearing perfume and the clothes and jewelry that encourage the men to seek marriage from the woman. All widows must observe this period of mourning whether they are young, old, free, servant, Muslim or disbeliever, as the general meaning of the Ayah indicates.
Allah also said:
[فَإِذَا بَلَغْنَ أَجَلَهُنَّ]
(...then when they have fulfilled their term) meaning, when the `Iddah finishes, according to Ad-Dahhak and Ar-Rabi` bin Anas.
[فَلاَ جُنَاحَ عَلَيْكُمْ]
(there is no sin on you) Az-Zuhri said, "Meaning her Wali (guardian).''
(if they (the wives) dispose) meaning, the women whose `Iddah has finished. Al-`Awfi said that Ibn `Abbas said, "If the woman is divorced or if her husband dies and then her `Iddah term ends, there is no sin that she beautifies herself, so that she becomes ready for marriage proposals. This is the way `that is just and honorable'.'' It was reported that Muqatil bin Hayyan gave the same explanation. Ibn Jurayj related that Mujahid said:
[فَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا فَعَلْنَ فِى أَنفُسِهِنَّ بِالْمَعْرُوفِ]
(...there is no sin on you if they (the wives) dispose of themselves in a just and honorable manner.) "refers to allowed and pure (honorable) marriage.'' It was also reported that Al-Hasan, Az-Zuhri and As-Suddi said the same.
[وَلاَ جُنَاحَ عَلَيْكُمْ فِيمَا عَرَّضْتُم بِهِ مِنْ خِطْبَةِ النِّسَآءِ أَوْ أَكْنَنتُمْ فِى أَنفُسِكُمْ عَلِمَ اللَّهُ أَنَّكُمْ سَتَذْكُرُونَهُنَّ وَلَـكِن لاَّ تُوَاعِدُوهُنَّ سِرًّا إِلاَّ أَن تَقُولُواْ قَوْلاً مَّعْرُوفًا وَلاَ تَعْزِمُواْ عُقْدَةَ النِّكَاحِ حَتَّى يَبْلُغَ الْكِتَـبُ أَجَلَهُ وَاعْلَمُواْ أَنَّ اللَّهَ يَعْلَمُ مَا فِى أَنفُسِكُمْ فَاحْذَرُوهُ وَاعْلَمُواْ أَنَّ اللَّهَ غَفُورٌ حَلِيمٌ ]
(235. And there is no sin on you if you make a hint of betrothal or conceal it in yourself, Allah knows that you will remember them, but do not make a promise (of contract) with them in secret except that you speak an honorable saying. And do not be determined on the marriage bond until the term prescribed is fulfilled. And know that Allah knows what is in your minds, so fear Him. And know that Allah is Oft-Forgiving, Most Forbearing.)