要写一篇关于“我忘了今天是星期六”的英语作文，你可以遵循以下步骤：

确定时态：

由于描述的是过去发生的事情，你应该使用过去时态。例如：“I forgot it was Saturday today.”

构建句子结构：

你可以从一个简单的句子开始，然后逐渐增加细节。例如：“I went to school this morning, but to my surprise, the school was closed.”

添加细节：

在句子中加入更多细节来丰富你的作文，比如你当时的感受或者你后续的行动。例如：“I was surprised and wondered why the school was closed.”

使用过渡词：

使用过渡词使文章流畅，例如：“However,”或“But,”来连接你的想法。

总结：

在作文的结尾部分，简要总结你的经历或感受。例如：“I realized that I had forgotten it was Saturday and decided to check my calendar when I got home.”

综上所述，一个简单的英语作文例子可以是：

Title: I Forgot It Was Saturday

Today, I woke up and went to school as usual. However, to my surprise, the school was closed. I was puzzled and wondered why. It was then that I remembered that I had forgotten it was Saturday. I felt embarrassed and decided to check my calendar when I got home to confirm the day. This little oversight reminded me to be more mindful of the days of the week.

希望这个例子能帮助你写出一篇关于“我忘了今天是星期六”的英语作文。