Day 14 but it varies
In an idealized 28-day cycle ovulation
occurs 14 days after the first day (cd 14) of the last mentrual period.
In real life, most of us have seen apparently normal ovulations between day 11 and maybe 15-16. For women with regular but longer cycles, let's use 35 days as an example, best guess is usually 35-14= cycle-day 21. With irregular cycles, there is no general reliable rule of thumb.