PMPro Adjust start date to the first following Sunday.

my_pmpro_checkout_start_date.php

<?php //do not copy

/*
	Adjust start date to the first following Sunday for a specific level
*/
function my_pmpro_checkout_start_date( $startdate, $user_id, $level ){

	if( $level->id == 8 ) { //Change to the preferred level you want this to apply to

		//which day is it
		$checkout_day = date("N");
		
		//days to sunday
		$days_to_sunday = 7 - $checkout_day;
		
		//add the days
		$startdate = date("Y-m-d", strtotime("+ " . $days_to_sunday . " days"));	

	}

	return $startdate;
}
add_filter( 'pmpro_checkout_start_date', 'my_pmpro_checkout_start_date', 10, 3 );