Learn Spanish (Latin America): El lunes participé en el club.

The accent mark tells you this is the completed past tense ("I participated"). For regular -ar verbs, the "I" form in this past tense always ends in an accented -é. The accent is important for both pronunciation and meaning!

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

It is built right into the verb! Because the -é ending on participé is only ever used for "I" in this past tense, you don't need to say yo (I). You could say Yo participé, but dropping the pronoun is much more natural in Spanish.

Why does the sentence use El lunes instead of a word for "on"?

In Spanish, you never use a preposition like "on" for days of the week. Instead, you just use the definite article el (the). So, el lunes automatically translates to "on Monday," and los lunes would mean "on Mondays."

Is the word club exactly the same in Spanish as it is in English?

Yes, it is a loanword borrowed directly from English. In Latin American Spanish, it is spelled the same and usually pronounced almost exactly like the English word, though the "u" might sound slightly softer.