“在秋季”，说 in fall 和 in the fall 都可以。其他季节也是如此。例如：

in fall:

1. These birds migrate northwards in spring and southwards in fall.  

这些鸟春天向北迁徙,秋天向南迁徙。

来自《简明英汉词典》

2. Grasshoppers die in fall.  

蚱蜢在秋天死去。

来自《简明英汉词典》

3. It doesn't rain a lot in fall, but it rains a lot in spring.  

秋天不下很多雨, 但春天下很多雨。

来自辞典例句

in the fall:

1. Life starts all over again when it gets crisp in the fall.  

秋天一到,天高气爽,生活就又重新开始了.

来自辞典例句

2. The leaves slowly rusted in the fall.  

秋天叶子慢慢变成红褐色了。

来自辞典例句

3. This flower seeds in the fall.  

这花秋天结实。

来自辞典例句

如果特指某年的秋季，须加the：

1. He was elected judge in the fall of 1991.  

1991年秋他当选为法官。

来自柯林斯例句

2. It took place in the fall of 1980.  

它发生在一九八零年秋季。

来自辞典例句

3. In the fall of 1943 false and malicious gossip was circulating.  

在一九四三年秋季,流传着一种捏造和恶意的闲谈。

来自辞典例句