Queen's BDay in Leiden or ??
 

We are flea market/junk fans. I know that in Amsterdam there are street sales all over town on April 30th. We'd prefer to base in a smaller town, though. Does this same tradition take place in other towns such as Leiden, Haarlem, or Utrecht? Other suggestions welcome. We'd be traveling around by train and bus- no car.

It is a national holiday and this article states that "Every city and town in the country transforms itself into a fairground"..
http://www.whatsonwhen.com/sisp/inde...event_id=20757

this article confirms that it is celebrated in Haarlem:
http://www.haarlemshuffle.com/cityguide/topic.php?id=9

The important thing to find out is whether train timetables are affected by any celebrations...

The junque comes out in Haarlem. The celebration is not so wild as in Amsterdam so shopping may be easier. Central Amsterdam is absolutely mobbed if the weather is nice.

When the crowds get too thick in the train stations the authorities can lock them up. This happened in Amsterdam about 5 years ago. I don't think that the trams run in Amsterdam. There is no way that they can use the Damrak because it is packed solid with people from the buildings to the river.

Every town and city has one to a certain extent. In Utrecht it is the evening before Koninginnedag. Den haag has a free concert the evening before. If you are looking for bargains you are better off in somewhere like Haarlem, Utrecht or any other town around than in Amsterdam which just gets too crowded. Tradionally there are always lots of children selling things, or performing to collect money (in theory to buy a Mother's Day present but in practice of course for them selves), and you will see these more outside Amsterdam and Utrecht.
Even the smallest village has it's kleedjes markt on Koninginnedag, plus other festivities going on.