The IPO market is lighting its fireworks a bit early, with a whopping 17 companies planning to list this week on U.S. exchanges.

Driving the news: Chinese ride-hail company Didi is expected to be the week's top float, with plans to raise nearly $4 billion.

• Other big issuers should be cybersecurity company SentinelOne, Turkish e-commerce platform D-Market and doughnut chain Krispy Kreme.
• Many of this week's companies began prepping their IPOs late last fall.

By the numbers: This would be the third time in 2021 that U.S. markets saw 17 IPOs, following weeks in February and March, per Renaissance Capital. Before that, however, it hadn't happened since December 2006.

• Q2 2021 is expected to go down as the busiest quarter for U.S. IPOs since 2000, while June 2021 will be the busiest month since that same year.
• Renaissance Capital's Matthew Kennedy emails: "We're all drowning in work! Buy-side, sell-side, lawyers & advisors ... I know the virtual roadshows of some sizable deals are sparsely attended since fund analysts have only so much time. And we've definitely noticed an uptick in prospectus typos."

Part of this week's boom is about getting out before the July 4 holiday. Another part is companies trying to price before quarter-end, so they needn't provide another set of financials.

• But a top Wall Street banker tells me not to expect a July lull, with dozens of companies expecting to price. He adds that we should see a slew of new IPO filings over the next week or two.

The bottom line: The IPO window isn't just open. The glass has been smashed and the framing has been removed.