Early this year, as Monday is only the 25th day of May and summer will officially begin.
For literary agents, there's a long holiday weekend. After that, it's Book Expo America in New York City, and they'll be out of the office to attend the bash.
For you, the author in search of representation, it means longer waits to hear back if you submit a query during the busy time of holiday weekend/BEA.
Once all that dies down, there's the summer hours to contend with. There's nothing going on in the big publishing houses on Friday afternoons and the literary agents put up their feet as well. The really successful ones head off for their beach cottages in the Hamptons on Friday afternoons.
Again, you the author will be waiting longer to get a response to your query.
Not that it isn't a good time to submit. There's not much point in waiting for a particular, magical season for sending off your sample pages and pleadings. A process already slow runs that much slower. Some writers are put off by it and don't submit, so that's all to your benefit if the piles of letters that teeter on the agent's desk are a bit shorter and maybe your particular plot synopsis can stand out in a smaller crowd.