Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Answering your question, The Fatwa Center at Islamweb, issued the following fatwa:
It is not permissible to exclusively designate the last Wednesday of the month of Safar for fasting or for any particular supererogatory acts of worship. This is allocating a particular act of worship to a particular time without religious evidence, and this is an innovation. It was authentically reported that the Messenger of Allah (peace be upon him) said: “If anyone introduces in our matter something which does not belong to it, will be rejected.” (Al-Bukhari and Muslim)
Almighty Allah knows best.