When is the best time to tell your employer that you are shomer shabbat?

When do you think is the best time to mention to an employer that you are shomer shabbat? I definitely don’t do it during the interview phase. Usually, I mention it upon receiving an offer, but I have some friends telling me I should wait until I actually start work. As I am back in the job-hunting world, I should probably have a set rule for myself. Any advice?

Just for a deeper understanding into my situation, I am applying for marketing jobs, some are experiential which often involve being on site on weekends. However, most are Monday to Friday office jobs, some hybrid, others 5 days a week in the office.