early termination fee

You're going to have to pay for the full first month of service, so the worst case is $150 + (23 x 20), so $610.  You can cancel free prior to installation, but yes, once it is installed, the ETF applies.

Don't I have 24 hours after it is installed to cancel?

From https://www.directv.com/legal/directv-residential-terms-of-service/ :

You can cancel Your Service at any time prior to the Installation of Your Equipment for any reason.  If You do so, You will not be subject to the Early Termination Fee.  We will refund You the cost of Your Service as applicable. 

If You cancel Your Service after the Installation of Your Equipment, You will be charged an Early Termination Fee.

There's not anything here that provides for that 24 hours.  There may be some sort of right of rescission in your state that DIRECTV would have to comply with, but you'd have to research that.

Sending someone out to install the dish and wire it up to your receivers, etc. is expensive.  They're willing to amortize that over 24 months of service fees, but if you walk before then they want compensation for it.

FYI, I googled and AI said it was after installation for 24 hours. Then called Directv

and the Rep said it was 24 hours after installation.  I would think your post of the term of agreement would be the right answer.  Just shows the misinformation out there including from Directv reps.

Now really confused, as I just read a thread from 2023 where Juniper and Shannon both said you had 24 hours after installation.

It used to be a 24 hour window from install where there was no ECF if you canceled. But it seems at some point in the last couple of years after AT&T included the new co-owner and now fully selling of the rest of their share (so AT&T no longer owns DirecTV) that window has gone away.

I suspect that is a extension of changing their cancelation policy. DirecTV no longer prorates the bill and instead it goes through at the end of the service month. That changed back in 2019. Guess at some point they updated it so it didn't matter if you called within the first 24 hours as you could no longer be canceled within that 24 hours anymore.