Multiple reputable sources report that Ukraine launched a major, widely described as "large-scale" or "massive," counteroffensive against Russian forces in early June 2023, which is late spring by meteorological seasons.

• The BBC reports that Ukraine’s "long-awaited counter-offensive" began in early June 2023, detailing Ukrainian advances and offensive operations along several fronts in the south and east.
• The Institute for the Study of War (ISW) and other military analysis outlets describe the June 2023 operations as Ukraine’s main 2023 counteroffensive, involving multiple brigades and extensive Western-supplied equipment.
• Major news organizations (e.g., Reuters, The New York Times) characterize the June 2023 action as a significant or large-scale counteroffensive, confirming its scale and intent.

Because this substantial counteroffensive did in fact occur in spring 2023 (as commonly used in military and media descriptions, and calendar-wise at the very end of spring/early summer), Sacks’s prediction that “there’s going to be a massive Ukrainian counteroffensive in the spring” is best judged as right.