'Open-air' is how I've translated such event; I tend to steer clear of 'plein-air' as I understand it a term applied to a specific painting style

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Note added at 9 mins (2008-09-14 14:20:50 GMT)
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Let me rephrase that. It's not so much that I understand it as that, as that it can be taken to mean that; so I simply try to avoid misunderstandings. However, for a text such as e.g. a critical work, where the target reader may well be infomed in the subject, then I'd use 'plein air'.



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Note added at 12 mins (2008-09-14 14:23:57 GMT)
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"plein air (pl!nĆ ârĆ; Fr. ple neRĆ),
1. the open air, esp. the daylight of outdoors.
2. Fine Arts. the quality of light and atmosphere out of doors, esp. this quality as rendered in painting.
[1890–95; < F: lit., full air]

plein-air (pl!nĹârĆ; Fr. ple neRĆ), adj.
1. pertaining to a manner or style of painting developed chiefly in France in the mid-19th century, characterized by the representation of the luminous effects of natural light and atmosphere as contrasted with the artificial light and absence of the sense of air or atmosphere associated with paintings produced in the studio.
2. designating a painting executed out of doors and representing a direct response to the scene or subject in front of the artist.
3. (of a painting) having the qualities of air and natural light.
[1890–95; adj. use of PLEIN AIR]
—pleinĹ-airĆism,
—pleinĹ-airĆist, n."
( From: Webster's Unabridged Dictionary )

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"plein air or plein-air (plnâr, pl-nr)
adj.
1. Of or being a style of painting produced out of doors in natural light.
2. Taking place outdoors: plein air dining.

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[From French (en) plein air, (in) the open air : en, in + plein, full + air, air.]"
( From: http://tinyurl.com/6pqzg6 )

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Note added at 24 mins (2008-09-14 14:35:34 GMT)
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Re. your note: I'd say that possibly 'plein air' isn't such a widely used term in this context cf.. for example:

http://tinyurl.com/2unsh

Personally, for what it's worth, in your context, I'd use 'open air'