Forget Parking in Alexandria: especially Old City. Take the Metro to King Street Station and walk down King Street. Since you are coming from Frederick, park at Rockville Metro Station Take the Red Line to Metro Centre and either the Yellow or Blue Line to King Street Station.

It's an all day thing, but I would get there around noon. Earlier is OK, but there won't be that many people. More people will show as the day goes on. Do some shopping or sight seeing if you get there early. There are some vendors near the Alexandria Town Hall (between Royal and Fairfax on King Street). John Crouch is worth a visit since they were my "Highland Supplier" when I lived in DC.

I am no longer a resident of the WDC area, but would have loved to have hooked up.

Actually, I was thinking about Scottish Walk and wondering if there was a similar event in SE PA. Say Phoenixville?