UPDATE: You can view the Lions' preseason schedule here.
No official date has actually been set, but the release of the 2015 NFL preseason schedule appears to be nearing. I say that simply because the NFL has announced its preseason schedule around this time of year in the past. Last year, for example, the preseason schedule was announced on April 9, and it was released on April 4 the two years before that.
Whether it's announced this week or later in the month, we already basically know half of the Detroit Lions' 2015 preseason schedule. Based on past years, they will likely play at the Cleveland Browns at some point in the 2015 preseason, and chances are they will wrap up the exhibition schedule with a home game against the Buffalo Bills.
As always, the other two opponents in the preseason are more of a mystery. The NFL could really generate some storylines by having the Lions play the Miami Dolphins or St. Louis Rams given what has happened in free agency, but it's unclear what goes into those scheduling decisions.
In any case, we should find out about the Lions' 2015 preseason schedule very soon, and the 2015 regular-season schedule should also be announced later this month.