Please can you enlighten me on the importance of fasting and special Nawafil on the last Wednesday of Safar?

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

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 as this is allocating a particular act of worship to a particular time without religious evidence, and this is an innovation.