The expiry dates for commodity contracts are set considering the time & efforts required in case there is a delivery of the contract.
For e.g: The expiry of mega gold contract [1 kilo] & gold mini [100 gms] contracts are the 5th while the expiry of smaller gold contracts like gold petal [1 gm], gold guinea [8 gms] is the last day of the month. This is because to give/take delivery of the the mega contract involves higher time and effort as against giving/taking delivery of the smaller contracts of the smaller gold contracts. There are times when the gold has to be moved from Exchange’s one vault to another and moving the bigger gold denomination take longer considering the value associated with it. Similarly Silver Mega is settled on 5th while other smaller contracts of Silver are settled at the end of the month.
Most of the contracts that are cash settled expire at the end of the month.