Why do the Prophet's Birthday and Sham el-Nessim always coincide with Monday?

Celebrating "Sham Ennessim" and the Prophet's Birthday is not permissible. Sham Ennessim is an ancient Egyptian holiday, celebrated by previous nations including Jews and Christians, and it is not permissible for a Muslim to partake in the holidays of disbelievers. As for the Prophet's Birthday, it is an innovation (bid'ah) introduced by the Fatimids. Neither the Messenger of Allah (peace be upon him) nor his Companions nor the Tabi'un celebrated it. Celebrating it is considered an innovation in religion that Allah has not commanded.