Is it permissible for the Imaams of Masjids to call people to visit memorials for those whom they consider as martyrs, celebrate it and deliver sermons? It should be noted that there is intermixing between the sexes there.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If these memorials are built on graves, the visitor has to follow the guidance of the Prophet on visiting graves. The guidance of the Prophet on visiting graves, according to  Ibn Al-Qayyim is that when he wanted to visit the graves of his Companions he would supplicate Allaah The Almighty for them and ask Him to shower them with His Mercy and Forgiveness. This is the sort of visit that he ordained for his Ummah (nation). The Prophet, sallallaahu ‘alayhi wa sallam, ordered them to say while visiting the graves: "Assalaamu alaykum, O dwellers of the graves, Muslims and believers. We will join you - Allaah Willing. We implore Allaah to grant us and you wellbeing." [Muslim]

It should be noted that it is impermissible to raise a memorial, edifice, or anything else over graves. It was narrated on the authority of Jaabir that he said, "The Prophet, sallallaahu ‘alayhi wa sallam, forbade plastering graves, sitting on them or building over them." [Muslim]

Celebrations and delivering sermons there are all religious innovations and acts of disobedience to Allaah The Almighty, whether or not the memorials are built on graves.

It is even worse when these celebrations are accompanied by evildoing such as intermixing between the two sexes. It is impermissible to call people to such religious innovations or to attend them, whether the caller is an Imaam or anyone else. Instead, this Imaam should call to the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam, and not towards religious innovations or error.

Allaah Knows best.