Learn Spanish (Spain): El jueves leo el periódico en el parque. - On Thursday I read the newspaper in the park.

In Spanish, you do not use a preposition like "on" for days of the week. Instead, you simply use the masculine word for "the" (el or los). So, el jueves literally means "the Thursday" but translates naturally to "on Thursday".

jueves ends in an "s". Does that mean it translates to "Thursdays" in this sentence?

No, jueves is just the normal, singular word for Thursday. To say "on Thursdays" instead of "on Thursday", the word jueves would stay exactly the same, but you would change the article to plural: los jueves.

Why is the day of the week (jueves) written in lowercase?

Unlike English, Spanish does not capitalize days of the week, months, or languages unless they happen to be the very first word of a sentence. Here, the sentence starts with El, so jueves stays lowercase.

Where is the word for "I" in this sentence?

The verb leo (I read) already tells us who is doing the action. The word yo (I) is left out because the "-o" ending on the verb makes it completely clear that you are talking about yourself. This is very common in Spanish!