The crates will be available as a drop after Online Matches, and will unlock with 'Flowers' that you earn, or as a direct purchase via the Rocket League client (similar to buying Keys or Premium DLC Battle-Cars). Basically, it is the same system Psyonix has deployed in the past for other seasonal events that have revolved around things like Christmas and Halloween.
Alongside the Spring Fever Create will be the addition of 10 new items, as is accustomed with in-game events. These also can be purchased using the aforementioned 'Flowers.” Normally, getting enough of the event-based currency to unlock all of the event-based items, isn't very hard. You'll have to play a decent amount of matches, but nothing much more than you usually would play on a more regular occasion. Personally, I've never had trouble unlocking every item.Spring Fever will commence on Monday, March 19th, at 10 a.m. PDT. It will end on April 9th, at 5 p.m. PDT. For more details on how in-game events, Decriptors, and Event crates work, click here.