when is it??
The deadline is already past. It was yesterday (April 17th).
In fact, to comfortably have covered payments by yesterday, you'd have to have sold 1+ weeks ago. That lines up pretty well with that bottom in the low $6,000s. Quite a coincidence.

You realise that not everyone leaves this stuff to the last minute, right? Some will have sold in Jan in anticipation of paying taxes.
What's your point? Some people
do wait until the last minute, myself included. I literally sent in my tax payments yesterday.
And according to the tax-as-sell-pressure theory, even one person waiting until April means net sell pressure on the price leading up to the deadline.
You have to consider trading sentiment too. Most market participants hold onto losing investments. They don't realize losses until they absolutely have to. Because of the downtrend, it's likely many taxpayers held off selling until more recently. Think about it. Anyone who took profit in 2017 but reinvested the capital (tempting in a volatile market like BTC) needed to sell to cover the 2017 tax liability. Combine that with the downtrend market psychology I mentioned.