Is it permissible, in Sharia, for a charitable organization to sell tickets in advance (a month before the event) for dinner parties or charity meals to raise money for orphans? And does this fall under the ruling of selling food before taking possession of it?

The basic principle is that it is impermissible to sell meals before they exist, due to not owning them and the Prophet's (peace be upon him) prohibition of selling what you do not possess and selling food before taking possession of it. Exceptions to this are the salam and istisna‘ contracts, provided certain conditions are met. For the salam contract, it is stipulated that the full price be paid at the contracting session, that the food be delivered at a known future date, and that the food be describable in terms of its characteristics. As for the istisna‘ contract, it does not require the full price to be paid at the contracting session, and it is valid even if another party produces the item. The food must be clearly described to prevent dispute. Selling tickets for entry to a celebration venue in advance is permissible, as it is a rental contract (ijarah (definition)) where the benefit does not have to be immediately available at the time of the contract.