Before deliveries can be assigned, the clearing house needs to know who is standing for delivery. On first position day each clearing-member files the long positions in its book by the date they were established, because delivery is generally assigned to the oldest long.
For traders the day matters mainly as a deadline signal: it is the last quiet moment before first-notice-day, and most brokers begin liquidation-only treatment of retail accounts around here.
Example: a contract with a first notice day on 30 November has a first position day on 26 November. A speculator still long on the 26th will typically get a phone call, and an automated liquidation on the 27th if there is no reply.
Related: first-notice-day, delivery-notice, liquidation-only, clearing-member, delivery-period