Clicking on the "link" icon on crop planting schedules in the sidebar will toggle it "on" linking planting schedules together so you can move them all at once.

When turned "on" the link icon will turn black:
​
​

When you have linked multiple planting schedules together you are ready to move them:
​
​

Then drag and move any of the related planting schedules and once you move one the others will jump to move together with it.

Here is an example moving the third carrot planting:
​
​

When it is moved the others jump forward to join it:
​
​

Linking crop planting schedules only works within a crop category.

You cannot link planting schedules across crop categories (for example trying to link lettuce with carrot plantings).