The spring is the mischievous time of year because the gaps in the wall are found during spring.
The poem is about two neighbors who have a wall between their land. They have an agreement to meet once a year and maintain the wall, fixing any gaps that have developed in it over the course of the year. The speaker finds this strange or at least unnecessary, but his neighbor wants to continue the practice because, as he says, “good fences make good neighbors.”
Something there is that doesn’t love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast.
It makes sense that you would not find the gaps in the wall until spring. First of all, during the winter the wall is probably covered in snow and it is hard to look at it closely. Also, in the springtime the plants grow wild. It is this combination of erosion and plant intrusion that is likely causing the wall to fall apart, and this is the reason why it needs to be mended every year.
The gaps I mean,
No one has seen them made or heard them made,
But at spring mending-time we find them there.
So the spring is the most mischievous time of year, because between the rain, the snow melting, and the plants growing, the wall doesn't stand a chance! There will be holes all over it. They have to fix the wall every spring even though there are no animals on either farm. This is why the speaker does not want to bother with it. His neighbor thinks that having a good stable wall between them creates good boundaries in their relationship. It is a metaphor. It also does get them together once a year, which is more than some neighbors do I guess!