PenultimateTuesday.jl

using Dates

function findfirst_tuesday(d::Date)
    fd = dayofweek(firstdayofmonth(d))
    return Tuesday - fd + 1 + 7(fd > Tuesday)
end
findfirst_tuesday(m::Month, y::Year) = findfirst_tuesday(Date(m, y))

function penultimate_tuesday_date(m::Month, y::Year)
    ft = findfirst_tuesday(m, y)  # first Tuesday
    d = Date(Day(ft), m, y)
    n_tuesdays = daysofweekinmonth(d)
    pt = Day(ft + 7(n_tuesdays - 2))  # penultimate Tuesday
    return Date(pt, m, y)
end
penultimate_tuesday_date(d::Date) = penultimate_tuesday_date(Month(d), Year(d))

Ported and extended logic to PenultimateDays.jl!