〜とあって
because (it's a special occasion)
Plain form / Noun + とあって
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とあって gives a special or unusual situation as the reason for what you can see happening: 連休とあって、観光地はどこも混雑している, "because it's a long weekend, tourist spots are packed everywhere".
The first half is a notable circumstance, like a holiday, a celebrity visit, a sale or a first time. The second half describes a natural, observable result, often crowds or excitement. It's a favourite of news reports.
The speaker is describing the situation objectively, so the second half isn't a request, a wish or the speaker's own plan.
Compare だけに (N2), "as you'd expect from", which evaluates. とあって just reports cause and effect. Don't confuse it with とあれば, "if it's for".
Examples
連休とあって、観光地はどこも混雑している。
Because it's a long weekend, tourist spots are packed everywhere.
人気歌手が来るとあって、会場は満員だった。
With a popular singer coming, the hall was full.
初めての海外旅行とあって、娘は興奮している。
It's her first trip abroad, so my daughter is excited.
半額セールとあって、朝から長い行列ができた。
With a half-price sale on, a long queue formed from the morning.
久しぶりの晴天とあって、公園は家族連れでにぎわった。
With clear skies for the first time in ages, the park was full of families.