There's a good argument here that Zelensky and Ukraine know that this is the time to strike, as soon as mud season is done.One of the reasons the campaign in Ukraine turned into a quagmire, for both sides, is the inability to use air power by either side. The air defenses are keeping air assets well behind the front lines. Even if Ukraine gets the modern fighters, they'll have little impact on the situation on the ground. Cutting off the supply of the anti air systems would finish the war for Russia without a doubt.
They know they can't count on western weapons forever as well as know that they need to make some big strikes against Putin lest it turn into another Donbas stalemate.
Likely we'll find out in the next month or so.

The Coming Fight will be Different
How Ukraine's Next Offensives Will Differ From Its previous Campaigns