The Picardy Third is generally more common in Baroque period music, and perhaps rarer in the Classical period.

What is a Picardy Third? Here let me give a simple example.

In the first half here there is first a B diminished chord, followed by E7; we would naturally expect it to end on Am.

But in the second half of the example we see that in the same situation of Bdim -> E7, we suddenly end on an A major chord instead; this is the Picardy Third.