Poetic or literary term for 'this evening' or 'tonight'. In everyday speech, 今夜 (こんや) is more common.
The moon is beautiful this evening.
Let's stay here tonight.
今宵 has a literary, old-fashioned feel, whereas 今晩 is the common, neutral way to say 'tonight' in daily speech and greetings.
Compound of 今 (こ, 'this') and 宵 (よい, 'evening'). The reading こよい is a contraction of このよい.