The Kedusha of shabbos increases as the day progresses. בה, בו. בם. The Chasam Sofer explains that is why in the morning we make a kedusha rabba. There are three tefilos which the Tur explains is against three shabosos, shabbos of creatiion, shabbos of kabolas hatorah and all eternal shabbos. The afternoon is the holiest of the three having the sudah against all three.
