温暖的天用英语可以说成 "warm day" 或者 "mild day"。
"Warm" 是形容词，意思是"温暖的"，可以描述天气、气温等，例如：
The sun is shining and it's a warm day. (阳光灿烂，天气温暖。)
It's warm enough to go outside without a coat. (天气温暖，不用穿大衣就可以出门了。)
"Mild" 也是形容词，意思是"温和的"、"不热也不冷的"，可以描述天气、气温等，例如：
The weather is mild today. (今天的天气很温和。)
The mild temperature makes it a perfect day for a picnic. (温和的气温使得今天成为野餐的好日子。)
"Warm" 是形容词，意思是"温暖的"，可以描述天气、气温等，例如：
The sun is shining and it's a warm day. (阳光灿烂，天气温暖。)
It's warm enough to go outside without a coat. (天气温暖，不用穿大衣就可以出门了。)
"Mild" 也是形容词，意思是"温和的"、"不热也不冷的"，可以描述天气、气温等，例如：
The weather is mild today. (今天的天气很温和。)
The mild temperature makes it a perfect day for a picnic. (温和的气温使得今天成为野餐的好日子。)