Is marriage between the two Eids bad from an Islamic perspective? And does singling out the 15th of Sha'ban for worship have great merit mentioned in the noble Hadith?

The belief that marriage during a specific period leads to death is an innovation and a misguidance that contradicts Islamic Sharia in several ways:

Firstly: It claims knowledge of the unseen, whereas death and life are in the hands of Allah alone, and no one knows when they will die.

Secondly: It disparages faith in divine decree and destiny, for everything is written and predetermined by Allah.

Thirdly: It disparages monotheism and promotes pessimism (al-tira), which the Prophet, peace and blessings be upon him, forbade and considered a form of shirk (polytheism).

Indeed, the Prophet, peace and blessings be upon him, married Aisha, may Allah be pleased with her, in Shawwal, and she was among his most beloved wives. This refutes the claims of pre-Islamic ignorance and pessimism regarding marriage in this month.