Short answer: it is not a clear yes or no. Some people do a psilocybin ceremony while on citalopram, and responses vary. Effects can be dampened for some, while others still have a noticeable experience. I cannot verify safety for your specific situation.
Key factors are your current stability, your citalopram dose, and how long you have been taking it. Do not stop citalopram abruptly to attend a ceremony. Discuss your plan with your prescriber and with an experienced facilitator, and prioritize screening, preparation, safety, and integration.
Common routes discussed with a prescriber include: continuing citalopram and accepting a chance of a less intense effect; tapering first, then waiting roughly five half-lives of citalopram, about eight days, while noting that withdrawal could still influence the session; or a slower taper with time to stabilize before scheduling, which is often more realistic after long term use.
For general context about what a ceremony involves and how to prepare, you can read this psilocybin ceremony overview: psilocybin ceremony.
I hope this helps you weigh your options with proper guidance.