Refers to a 24 hour period other than a calendar day. The start time, when different to midnight, will be chosen for operational reasons. In many countries the gas day runs from 06:00 to 05:59 on the next calendar day.

06:00 was chosen in the UK as it falls at the end of the low demand night period and before the beginning of the morning peak. It also means that it is relatively easy to balance the day’s volumes during the quiet nighttime period.