While traditionally flights were cheaper when booked round trip and 21 days in advance, those requirements are often no longer required. I just did a month long search on ITA for one way flights from New York to Los Angeles. There are plenty of tickets for next Saturday (7 days from now) for $130. The round trip price is simply the two one way tickets added together.

Within the next 7 days, the price increases but not by much. You can travel anytime before Saturday for only $284 one way. Obviously if you want a refundable/changeable ticket the price is more expensive.

While this is only one example (and a domestic USA itinerary only) the point is that the days of requiring a round trip, with a Saturday night stay and 14 or 21 day advanced purchase are often no longer required.