There is no 'safe day.' I can speak from the previous admissions cycle that everything seemed to be somewhat random. My first interview was Oct 22 (and I think that was one of the first interview dates offered, maybe one or two other programs were before that). My last date was Dec. 10, and I think a few interviews went into the next week. All of my interviews were on Thurs - Fri - Sat (NO Sun) and 1 on Monday.
You will find that some programs like to space out their interviews (eg. every Thursday for 4 weeks), some like to random (eg. Saturday in Oct and Monday in Dec) and some like to make a weekend out of it (eg. Thurs, Fri, Sat all in the same week).
I personally got together with another applicant at my school and made a google calender of the interview dates that were available on the SF match website in June. Turns out that it wasn't extremely helpful, but if there are a few programs you would give up any interview date for a chance to go there, it helps you choose another date for another program and essentially 'save' that place on your calender. It also helps show you the dates that will be popular and avoid those at all costs (the Friday before ARVO, the Friday before Thanksgiving).