English Translation
I’ve forgotten whether it was Saturday or Sunday.
Some of you may feel 土曜だったか日曜だったか redundant. If I literally translate the Japanese expression, it would be “whether it was Saturday, or whether it was Sunday”, which sounds awful in English. But actually we, Japanese native speakers, sometimes reduplicate だったか like this.
You can simply rephrase like this:
それが土曜か日曜か忘れました。
But the nuance of 土曜だったか日曜だったか is “I’ve COMPLETELY forgot” or “I have NO IDEA which one was that”.