Does Islam permit a solemnization ceremony to be held in a mosque?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
A mosque or the term ‘urf can be defined as a place prepared specifically for prayer forever. 
Basically, holding the solemnization in a mosque or any suitable place is permissible. Regarding solemnization in a mosque, Islam permits this. This can be seen as stated in a hadith, where the Prophet PBUH acts as the officiator in the solemnization of a woman in a mosque.
From Aisyah R.Anha, Rasullullah PBUH said:
أَعْلِنُوا هَذَا النِّكَاحَ ، وَاجْعَلُوهُ فِي الْمَسَاجِدِ ، وَاضْرِبُوا عَلَيْهِ بِالدُّفُوفِ
“Make this marriage publicly known, solemnise it in the mosques, and play tambourines in honour of it.” 
Imam al-Munawi stated that this hadith shows that solemnization in a mosque is permissible. He said, in this hadith, it shows that holding a solemnization in a mosque is not disliked, different from trade and other activities similar to it. 
According to this hadith, the majority of scholars has ruled it permissible to hold a solemnization ceremony in a mosque to attain blessing and inform the public of the marriage. 
Ibnu Hajar al-Haitami cited the opinion of al-Baghawi and others that holding a solemnization in a mosque is sunnah in a marriage. 
According to the question asked, in our opinion, it is permissible to hold a solemnization ceremony in a mosque. However, the ethics and manners, as well as the rules set in a mosque, should be followed.
Among the manners are as the following:
- Perform other sunnah worships such as ablution, intention for iktikaf and performing the two rakaat sunnah tahiyyatul masjid prayer before sitting down.
- Follow the manners of dressing such as wearing clothing that is pure, clean and covering the aurah.
- Be mindful of the manners in dressing up and adornments as to not be excessive or exaggerated.
- Be mindful of the prohibitions such as the prohibition of entering the mosque for those who are in major hadas (major impure state for both men and women), which includes menstruating women.
Department of Islamic Development Malaysia (JAKIM) has also published a guideline titled Solemnization in Mosques and Matters Related to It. 
Furthermore, we would also like to suggest having or creating a special place or section not in the prayer hall for the solemnization ceremony to be held. This is to make it easy for those who are in major hadas to also attend the ceremony.
To close, we include the decision made by the 95th National Fatwa Committee Council which convened on 16th – 18th June 2011. The Muzakarah discussed the Ruling of Solemnization Ceremony in Mosques and decided the following:
In the Muzakarah’s opinion, the Jumhur of scholars has acknowledged that solemnization is permissible to be held in a mosque. Moreover, scholars of madhhab Syafie are of the opinion that it is sunnah and encouraged to hold the solemnization ceremony in a mosque.
Hence, the Muzakarah has agreed that holding a solemnization ceremony in a mosque or surau is sunnah and will be blessed. However, people from both the groom and bride who attend the ceremony must follow all the manners and rules of when being a mosque or surau. 
May Allah SWT grant us understanding in this religion. Amin.
 Narrated by al-Tirmizi (1089) and al-Baihaqi (15095) in al-Sunan al-Kubra. Imam al-Tirmizi said: This is a Hasan Gharib hadith. This is a hadith evaluated as dhaif as stated in Dhaif al-Tirmidzi (1089) except on the phrase “أَعْلِنُوا”.