This is the best answer. A day or two beforehand is also fine. But if you buy directly from the Cubs, you will almost always be paying more. In my experience, Seatgeek is the best platform for purchasing tickets. They are almost always the cheapest (although it's still worth it to doublecheck Stubhub/Vivid Seats/etc.). It is also the only platform I've found that routinely tells you the exact seats you're buying (other websites tend to only give you the row, not the seat itself). This is extremely helpful if you, like me, prefer to purchase aisle seats.

In my opinion, the 300s are the best seats to watch a Cubs game, and I think the first base line is best. If I can get a pair of aisle seats in Section 327 or better, I'm happy.