Learn Spanish (Latin America): Mil personas llegan al museo el lunes. - One thousand people arrive at the museum on Monday.

In Spanish, you don't use un before mil. You just say mil to mean "one thousand". If you wanted to say "two thousand", you would specify the number: dos mil.

Why does the sentence use al museo instead of a el museo?

Whenever the preposition a (to/at) is followed directly by the masculine article el (the), they must combine to form al. So, a + el = al.

How come el lunes means "on Monday"? Where is the word for "on"?

In Spanish, you don't use a preposition like "on" with days of the week. You simply use the article el (the) before the day. Literally it says "the Monday", but in English we translate this as "on Monday".

Why does the verb llegan end in "-an"?

llegan comes from the verb llegar (to arrive). Because the subject of the sentence is mil personas (a group of people, which acts like "they"), the verb takes the "they" ending, which is "-an" for "-ar" verbs.