I have heard that too, yet the four Gospels seem to infer that the beginning of the day is at sunrise.
In the end of the sabbath, as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre. Matthew 28:1
And very early in the morning the first day of the week, they came unto the sepulchre at the rising of the sun. Mark 16:2
Now upon the first day of the week, very early in the morning, they came unto the sepulchre, bringing the spices which they had prepared, and certain others with them. Luke 24:1
The first day of the week cometh Mary Magdalene early, when it was yet dark, unto the sepulchre, and seeth the stone taken away from the sepulchre. .John 20:1
KJV
More help:
Gen 1:5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
(NOTE: One 12 hour evening 6pm until 6am. Then one 12 hour morning 6am until 6pm. An evening & a morning = One 24 hour DAY)
Lev 23:32, the Lord said, "From even unto even shall ye celebrate your Sabbath."
(NOTE: Sabbath Day; Starts with evening & ends with evening = one 24 hr day)
Old Testament Jewish doctrine only recognized 3, 4 hour watches: The 1st watch - beginning of the watches (Lamentations 2:19). The middle watch (Judges 7:19). The morning watch (Exodus 14:24 & 1 Samuel 11:11)
These began at 6pm/sunset until 10pm, 10pm until 2am & 2am until sunrise/6am
During Roman rule the number of watches changed to 4. Jews, Greeks & Romans divided the night 4, 3 hour watches. Each watch marking the period for which sentinels or pickets were on duty.
These began at 6pm/sunset & were 3 hours ea. Simply referred to as 1st watch/evening, 2nd watch, 3rd watch & 4th watch.
Matt 14:25 And in the fourth watch of the night Jesus went unto them, walking on the sea.
(NOTE: NT watch terms - even (evening) - 6pm til 9pm, midnight - 9pm til midnight, cock crowing - midnight til 3am & morning 3am til 6am
(NOTE: The FOURTH WATCH)
John 11:9 Jesus answered, Are there not twelve hours in the day? If any man walk in the day, he stumbleth not, because he seeth the light of this world.
(NOTE: Jesus says; There are 12 hours in a day. This doesn't include evening/night)
Find additional info here:
https://www.simpletoremember.com/articles/a/jewish-time/
http://torahcalendar.com/HOUR.asp