It's deffo 26 weeks, but sticking to actual dots on the calendar really isn't important - it's not as if every baby wakes up on that day desperate for solids.
It's 26 weeks/6 calendar months because that's been the date which researchers have used in their studies, comparing babies who have had solid food before or after this date. We can pretty much assume that the babies in the studies have not all started on the dot, and some have been a bit before and some have been a bit after.