Summer has traditionally been a slow time for the major broadcast networks in the U.S., as well as for TV audiences. With viewing numbers down, and the key Fall sweeps coming up, summer has traditionally been a dumping ground for program repeats, a place to test new programs, and a last chance to air contracted (but unaired) episodes of canceled series. Outside of the occasional big sporting event, there's not much to look forward to on broadcast television.
On the other hand, the lack of quality competition from the big networks gives newcomers an opportunity to counter-program with the best of new programs and episodes. When Fox started, it moved up the starts of some of its better series to the summer. Putting original episodes of quality programs up against the dregs of big network offerings, gave viewers an incentive to sample and evaluate Fox series and build audiences.
Spanish-language network Univision has been trying to move from a niche network to a challenger to the Big 4. It's expanded its reach beyond urban areas with high numbers of Hispanics, adding its own stations in larger markets, picking up affiliates, and making a push to get on multichannel basic service tiers. It's worked to shift its programming focus from airing licensed series from other Latin American networks and channels, to a mix reflecting its goal of being a general-interest broadcaster.
Of course, Univision's ability to outdraw the Big 4 is not only a result of programming acumen. It's been helped by two long-term trends: declining ratings and shares for the broadcast network as viewers have shifted to and expanding number of viewing alternative; and the fact that Hispanics are the fastest growing demographic group in the U.S. Still, their successes over time suggest that they've made the move from being a niche service to becoming a fully competitive general-interest network.
Source: Univision is the #1 Network for the Second Consecutive July Sweep Among Both Adults 18-49 And Adults 18-34, TV by the Numbers.