Shabbos morning is called kedusha rabba. Explains tne Chasam Sofer that the kedusha increases in the morning over the night and so on. We say vo, voy and wom. We can explain this with the Tur that the three tefilos of shabbos are against three shabbosim. Friday night is against shabbos bereishis, of creation. The morning is kabolas hatorah and mincha is yom shekula shabbos, the time of techiyas hamesim.
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