1(fig.) a time of renewal and flourishing
2(lit.) springtime when the weather is warm and flowers are in bloom (idiom)
Example
bù zhī不知hé shí何时chūn nuǎn huā kāi春暖花开，jīn tiān今天zǎo chén早晨hái还xià xuě下雪le了。
不知何时春暖花开，今天早晨还下雪了。
I don't know when spring will come and when the flowers will bloom. It was still snowing this morning.