You have to do it by date, not weeks - because there aren't exactly 4 weeks in every month. She'll be 6 months on the 27th. But you don't have to wait for the exact date, it can be whenever they're ready in terms of sitting up etc
A month is a month no matter how many days in it. So 27th will be 6 months. A week or so won’t make a difference if they are ready. It’s not like suddenly 3 days before they are 6 months it changes. It’s a guideline to when babies are normally ready
My bubs is also may 27th🥳 we will be starting solids around the end of the month x
I count the day there born, so mine is the 22nd, so she's 5m 1w, and 2d old. I'd slowly start foods at 6 months, and always remember, food is fun.