I know that sign-up sheets for Chorus Calls go up one week before the audition in NYC and LA, but if the call is taking place on a Saturday (or, if one week before the ECC, the Equity Office is closed), when will the sheets be posted?

The sheets will be posted at 9:30 AM on the next business day that the Equity office is open. All information on audition procedures can be found via links on the Casting Call search page In Chicago, no sign-up list is posted in advance. Sign-up for ECCs in Chicago begins 30 minutes before each call.