Use ～折に in formal writing and polite speech to mean "on the occasion of / when the opportunity arises," typically in business letters, email openings and closings, and greetings. It carries a warmer, more courteous nuance than ～際に and is favored for pleasant or neutral occasions — avoid it for accidents, mistakes, or emergencies. It very often appears as ～折には or ～折は when setting up a polite request or invitation.