Sir the date is fixed due to Sankranti (Kanya) factor. The date may change after some years earlier it was 16th Sep.
The festival Vishwakarma Jayanti is celebrated almost entire north peninsula off course in some parts of country it is celebrated next day of Diwali along with Govardhan Puja(Annakut), Kartik Shukla Padyami, the first day of the lunar New Year in Gujarat.
It is very interesting that on this day and specially in Eastern Region including EUP/CG NonVeg and Liquor also offered to Deity.