Is it permissible to perform I'tikaf starting from the night of the 25th and thereafter? And what is the ruling on someone who performed I'tikaf on the nights of the 26th and 27th, then left due to work necessity?

It is permissible to perform Iʿtikāf (spiritual retreat) in the mosque whenever a Muslim wishes, and its minimum duration is one hour. It is permissible to perform Iʿtikāf on any night and at any time, and there is no specific time required for its beginning or end if it is a voluntary act. There is no harm for the person in Iʿtikāf to leave their retreat for work or any other purpose if they did not intend to continue the Iʿtikāf for the remainder of the month; however, if they did intend to do so, they are not allowed to leave, otherwise their Iʿtikāf becomes void.