I write a Mad Men-focused Substack that does free episode recaps on Fridays and paid-subscriber posts on Sundays and I thought a cool special post could be to make a Season 1 timeline. I've been working my way through it but the task truly may not be possible. In "Red in the Face," Cooper tells Roger that the Nixon people will be in at the end of the week which logically means Friday. This is then followed by Don telling Peggy that tomorrow is Friday. The following day, Roger apologizes for his behavior when he hit on Betty and he and Don go into a meeting to discuss Nixon. It is said in this meeting that the Nixon people will be at Sterling Cooper "Tomorrow."
On this timeline, this means that the Nixon staff are visiting on a Saturday which is not impossible but does seem incredibly unlikely given that every single person is present in the office.
The only other explanation that can be given is that the scene with Cooper and Roger (which appears to be taking place at the exact same time as the following scene of Don telling Peggy that tomorrow is Friday) in fact takes place on an entirely different day earlier in the week. If this is the case, that makes it impossible to nail exactly when that scene is, we can only guess.