Yes, there is now. There’s been an official statement about it.
The previous gaps between season 18&19, 19&20 and 20&21 were all 12 days.
As such, 13th day will not be the season start.
Having only a few day gap from patch deployment to season start has several times already bitten Blizzard’s rear end. And with the issues plagueing PTR in mind do you really want to risk botching the season from the start?