If possible, buy your tickets on Tuesday or Wednesday.
This is separate from advice about what days to actually travel: plane fares fluctuate in weird ways, and prices for any given flight tend to be lowest on Tuesday and Wednesday, okay on Thursday, and worst on the weekends.
Also, most airlines still charge significantly less for a round-trip ticket than for two one-way tickets. (For complicated reasons, we just bought two one-way tickets to another city, and one one-way ticket back; I did it this way to be completely sure we would be traveling together on the flight out. But it probably would have been cheaper as a round-trip for me and a one-way ticket for the partner.) Air Canada is an exception to this (I don't know where you're going, so that may or may not be relevant.)