Considering the pluralities of Heb 5:7....:Who in the days of his flesh offered up prayers and supplications..... this obviously was not the first or only time He prayed such, or even the first time in the garden of Gethsemane for that matter.
Well, of course it's "days." It would sound very strange to say "day of His flesh," so it's an idiom referring to His human existence. I see no problem here. He prayed three times in the Garden, and that fits Hebrews 5:7 easily.
1 When Jesus had spoken these words, he went forth with his disciples over the brook Kidron, where was a garden, into which he entered, himself and his disciples.
2 Now Judas also, who betrayed him, knew the place: for Jesus oft-times resorted thither with his disciples. Jn 18