When planning outdoor special events, always plan for a rain date
The DC area has had rain for more than one week ("Washington slogs through storms as rain continues for a sixth day," Washington Post), and this will continue for a few more days.
Mostly, those organizations that scheduled outdoor events during this time are screwed, such as the Garden Tour in the Shepherd Park neighborhood.
Always include a rain date when planning such events, if they are to be held mostly outside.
This does pose a challenge. Note that the Lynchburg Art Festival poster pictured at left had its rain date on its promotional poster (which is what should be standard practice).
But it raises other questions.
Do you merely schedule the day after as the rain date? What if there is a weather front lasting for awhile?, such as is the case this week in DC.
In that case, the next day might be just as bad as the first.
On the other hand, if you postpone the event for a week, if there are vendors, many might not be able to participate if they have prior commitments.
OTOH, you'll have more participants and a better experience, if you postpone, but it will still pose challenges, making it harder to pull off the event as planned and anticipated.