While it's generally a good idea to avoid re-using the same word in one sentence or paragraph, I don't think that applies here. You have a parallel construction, you're talking about the same thing happening twice, and this is more clear if you use the same word.
For example, consider this sentence: "When Tuesday arrived I arrived at home to find that a letter had arrived." Clearly bad. I'm using "arrived" to talk about three different kinds of arrival. The sentence sounds repetitive and confusing.
But, "Two letters arrived on Monday and a third arrived on Tuesday." Now the kind of arrival in both cases is the same. You're indicating that a similar thing happened on Tuesday to the thing that happened on Monday. In this case, a parallel construction is good: use the same word. If you wrote, "Two letters arrived on Monday and a third came on Tuesday", the reader might think that you're deliberately creating a contrast: the letters on Monday "arrived" but the letter on Tuesday "came". Ok, in this case, it's unlikely to create much confusion, I think the reader will figure out that they're synonyms. But imagine a less obvious case. Like, "John registered on Monday, and Bob signed up on Tuesday." Now the reader might really wonder, Are "registering" and "signing up" the same thing or different things? Or you trying to say they both did the same thing, just on different days, or are you trying to establish a contrast between what they did?
To put it another way, avoid using the same word to mean two different things, or to apply in two different contexts, within a short space. But when you want to make clear that it is the same thing, use the same word.
"Two letters arrived on Monday, and a third arrived on Wednesday." Good. Clear parallelism.
"Two letters arrived on Monday, and a third came on Wednesday." Weak. Unclear parallelism.
"Two letters arrived on Monday and a third on Wednesday." Fine in this case. It is clear that you mean that a third arrived on Wednesday. In other contexts it might not be clear and you might need to specify.
"Two letters on Monday and a third on Wednesday arrived." Awkward. Normally prepositional phrases that modify a verb come after the verb. Imagine the sentence with just one day. "The letters on Monday arrived." You sound like Yoda. :-)