Can a woman who is in ‘iddah following her husband’s death attend her daughter’s wedding?
Question

                My father died on 3/1/2018, and my wedding was to be on 22/2/2018, but we delayed it until 7/4/2018, i.e., until the end of my mother’s ‘iddah. Is it permissible for her to attend the wedding preparations? Is it permissible to have a wedding at all, or is that contrary to Islamic teaching?
Praise be to Allah.Firstly:
The woman whose husband has died must observe mourning for her husband for four months and ten days, as Allah, may He be exalted, says (interpretation of the meaning):
“And those who are taken in death among you and leave wives behind - they, [the wives, shall] wait four months and ten [days]. And when they have fulfilled their term, then there is no blame upon you for what they do with themselves in an acceptable manner. And Allah is [fully] Acquainted with what you do”
[al-Baqarah 2:234].
Al-Bukhaari (1280) and Muslim (1486) narrated from Umm Habeebah (may Allah be pleased with her) that she heard the Prophet (blessings and peace of Allah be upon him) say: “It is not permissible for a women who believes in Allah and the Last Day to mourn for more than three days for anyone who dies, except for a husband, four months and ten days.”
Secondly:
What is meant by mourning is refraining from adornment and perfume, and not going out of the house by day except in the case of need, and not going out by night except in the case of urgent necessity.
Al-Bukhaari (4923) and Muslim (2740) narrated that Umm ‘Atiyyah said: We were forbidden to mourn for anyone who dies for more than three days, except in the case of a husband, (for whom the mourning period is) four months and ten days, during which we should not apply kohl or perfume, or wear any dyed garment, except a garment made of ‘asb (coarse, partially dyed cloth). A concession was granted to us when our menses ends and when doing ghusl, we could use a little costmary or azfaar (types of incense). And we were forbidden to follow funeral processions.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (8/130):
The woman who is observing ‘iddah may go out to tend to her needs by day, whether she is divorced or recently widowed, because of the report narrated by Jaabir who said: My maternal aunt was thrice divorced, and she went out to tend to her palm trees. She was met by a man who told her not to do that. She mentioned that to the Prophet (blessings and peace of Allah be upon him) and he said: “Go out and tend to your palm trees; perhaps you will give charity (from the yield) or do some good deed.” Narrated by an-Nasaa’i and Abu Dawood. Mujaahid narrated: Some men were martyred on the day of Uhud and their wives came to the Messenger of Allah (blessings and peace of Allah be upon him) and said: O Messenger of Allah, we feel lonely at night; can we spend the night at the house of one of us, then in the morning we will hasten to go back to our houses? The Messenger of Allah (blessings and peace of Allah be upon him) said: “(Stay up and) chat at the house of one of you, then when you want to sleep, let each one of you go back to her own house.”
[The recently-widowed woman] does not have the right [during the ‘iddah period] to stay overnight anywhere but in her own house, or to go out at night, except in the case of urgent necessity, because night is the time when mischief is expected to happen, in contrast to the day, which is the time when people tend to their needs and seek a livelihood, and they go and buy what they need. End quote.
Shaykh Ibn Baaz (may Allah have mercy on him) said: The woman in mourning whose husband has died must stay home and not go out, because the Messenger (blessings and peace of Allah be upon him) said to the woman whose husband had died: “Stay in your house until the prescribed time is over.” So the woman in mourning must stay home, and she should not wear beautiful clothes, or put on perfume or kohl, or wear jewellery.
There are five things that the woman in mourning must observe:

Staying at home until her ‘iddah ends
Not wearing beautiful clothes; rather she should wear clothes that are not beautiful, black or green or blue, provided they are not beautiful. 
Not wearing jewellery, gold, silver, diamonds, pearls and so on. She should not wear jewellery, and a watch is counted as jewellery, because it is worn for beautification and adornment.   
Not wearing kohl. So she should not apply kohl or put anything on her face that women use nowadays for adornment, apart from soap and water and the like.
Not wearing perfume; she should refrain from wearing perfume of all types, except when purifying herself following her menses.

She may go out when needed, such as to go to the courthouse or the hospital or the market.
End quote from Majmoo‘ Fataawa Ibn Baaz (22/199).
Based on that:
There is nothing wrong with you having the wedding at the appointed time, and there is nothing wrong with your mother attending it, if she avoids wearing adornment and perfume, because her going out in that case is going out for a need. But she must stay overnight in her own house.
And Allah knows best.