Approx. what date will summer 2019 high speed train schedule from Rome to Venice be available?

Joyful, just use current schedules for the same day of the week you'll be traveling. Schedules tend to change very little if they change at all. You just can't pre-purchase tickets until they become available.

New schedules should be out after June 8th but tickets may also become be available before that date.

I bought my June Trenitalia tickets weeks ago.

This is Italian Railways, they will be available when they are available. Even when there isn't a timetable change, tickets are only sold 120 days in advance, so September tickets will not be available until July.
As Kathy says, for planning use times in April or May, they are unlikely to change significantly.

And when they do appear, the cheap tickets will be available to everybody on the same date, nobody can "queue-jump".

For more info see here: https://www.seat61.com/Europe-when-do-train-bookings-open.htm

Good to know, Tim. I had checked some random June dates on Trenitalia for Termini> Venezia S.L. and tickets weren't showing as purchasable yet. Evidently they are for some other dates.. :O)

And don’t forget about Italo trains which also run that route.

Chris is wrong: Trenitalia sells tickets up to the next timetable change, not 120 days in advance. Tickets for September will go on sale on June 9, not in July.

This is the Italian railways, so You can bet they will meet the deadline has they have been doing for 70 years.

Italotreno published their summer ticket prices this morning 3/16/19. Purchased tickets this morning. Much higher than expected but got seat in their 1st class for $1 more than their coach class.
Thanks everyone!