Is it permissible to attend a wedding feast without an invitation?
Question

What is the ruling on going to a wedding feast without an invitation?
Praise be to Allah.With regard to attending a wedding feast without invitation, there are different scenarios:
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The first scenario is where the feast is only for those whom the host has invited, as is the case with many wedding feasts nowadays.
In this case, it is not permissible to attend except by invitation or with the permission of the host of the wedding feast.
Allah, may He be exalted, says (interpretation of the meaning):
{O you who believe, do not enter the houses of the Prophet, unless you are given permission [and are invited] to come for a meal; and do not enter in hopes of getting a meal and waiting for it to be prepared. But if you are invited, then enter} [Al-Ahzab 33:53].
Ibn Kathir (may Allah have mercy on him) said:
Then Allah makes an exception from that, as He says: {unless you are given permission [and are invited] to come for a meal; and do not enter in hopes of getting a meal and waiting for it to be prepared}.
Mujahid, Qatadah and others said: That is, not waiting until the food is ready. In other words, do not watch out for food, until it is almost ready, then you enter. This is something that Allah dislikes and condemns.
This indicates that it is prohibited to impose on others, which is what is known as being an uninvited guest. Al-Khatib al-Baghdadi wrote a book condemning those who impose on others, and told many stories about such people, which would take too long to quote here."(Tafsir Ibn Kathir  6/454).
It says in Sharh al-Khurashi ‘ala Mukhtasar Khalil (3/304):
A person who has not been invited should not enter except with permission. This means that if someone comes to the place where a wedding feast is being held without an invitation, he should not enter except with permission, and it is not permissible for him to enter, whether he eats or not. End quote.
It says in al-Mawsu‘ah al-Fiqhiyyah al-Kuwaitiyyah (20/335):
Imposing on people without an invitation:
It is not permissible for one who has not been invited to attend wedding feasts or other events, because doing so is unseemly and humiliating oneself, and that is not befitting for a believer…
The one who does that is called an uninvited guest or one who imposes on others.
Based on that, imposing on others is prohibited according to the majority of jurists, unless the uninvited person is a subordinate or attendant of an invited guest who is a prominent figure, and it is known that he does not usually attend events on his own. In that case, it is not prohibited, because the attendant is implicitly included in the invitation to the one whom he serves… End quote.
An exception is made if the one who comes without an invitation knows that the host of the wedding feast would be happy to see him, because of ties of friendship or kinship between them, or because he knows that the host is generous and is always happy to welcome guests.
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said:
… But if you know that your friend would be happy to see you, then this is not imposing on him; rather it is a type of kindness and humility on your part. There are many people for whom your coming without an invitation is something more beloved to him, and he will be very happy and will express his joy at that."(Ash-Sharh al-Mumti‘  15/428).
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The second scenario is where it is the custom for the host of the wedding feast to give permission to everyone, and he does not prepare food only for those whom he has invited. In this case, there is nothing wrong with one who was not invited attending the feast and eating.
Ibn al-Qayyim (may Allah have mercy on him) said:
Islamic teachings say that implicit permission is like explicit permission."(Madarij as-Salikin  2/1019).
It says in Mughni al-Muhtaj (4/410):
It is prohibited to impose yourself, which means attending the wedding feast without being invited, unless you are certain that the host will be pleased with that, because of strong ties of friendship.
Imam [al-Juwayni] restricted that to when the invitation is limited to certain people.
But in the case of an invitation that includes everyone, such as when the door is open for whoever wishes to come in [“open house”], then that is not regarded as imposing on others. End quote.
And Allah knows best.