This is reference to Kav's July 1st calendar entry, see Phillip Bump tweet below. That, and July 7th reference to 'Columbia' (Country Club) seem the most possible dates for the gathering where the assault occurred:
Daniel Dale
@ddale8
Asked by Booker if his weekday calendar reference to "skis" was a reference to brewskis, alcohol, Kavanaugh says he can't answer yes or no, has to explain. Booker moves on.
@TheRealEvanS
Replying to @ddale8
but only after he nodded and said 'yeah'
@ddale8
Sorry, didn't hear the "yeah":
Philip Bump
@pbump
That key July 1 calendar entry recorded by Kavanaugh came up by the Republican prosecutor. Despite how it overlapped with Ford's allegations, the prosecutor let him walk away from it.
Analysis | Kavanaugh is pressed on the key July 1 entry in his calendar. But only to a point.
washingtonpost.com