折に means 'on the occasion of' or 'when'. It is a formal expression commonly used in written language or polite speech. It often appears in set phrases like 折に触れて (from time to time) and 折に伺います (I will visit when the occasion arises). The nuance is more polite and literary than とき, suggesting a recognized opportunity or turning point. Avoid using it in casual conversation; it would sound stiff or overly formal. Note that 折に attaches to past tense verbs to mean 'when I did something', as in 伺った折に, and to nouns with の, as in お祝いの折に. It is not used with present or future tense verbs in the main clause, but the subordinate clause can use past tense form even for future events.