While the other answer is helpful, it doesn't answer your question: assuming the opera is not sold out, you can get the tickets for the same price on the evening before the performance as now. So there is no such thing as early-bird pricing or something similar.
However, you should buy the tickets now. Even for this week, which is a regular week as opposed to the week in which you'll be visiting, performances are mostly sold out or have less than 20 tickets available (at least according to the online booking system). Given the number of seats, this is almost sold out, and if you want to have a choice or sit together with others, you'll be almost surely be out of luck.
If you really want to be flexible, you could however go for the standing stalls: Tickets for these are just sold right before the performance (although people start queuing up quite a few hours earlier) and are cheap (but you have to stand of course).
Sometimes on the evening of the performance there are people right outside the opera house, trying to sell their ticket. But usually demand is bigger than supply.