Shticky Guy:- When the Torah refers to Pesach in conjunction to Nisson, it is exclusively referring to afternoon of Nisson 14th. Thereafter the Torah calls it Chag haMatzos, as indeed we do in our prayers.
Hence albeit we socially refer to the week of Pesach as “Pesach” rather than “Chag haMatzos” it technically is inaccurate. However, by Pesach Sheni, we somehow seem to do it the correct way, referring to the Zeman Shechitas hePesach as “Pesach” as does the Torah in conjunction with “Nisson 14th” being called “Pesach laShem”.