In month of ramadhan, most people have their fasting without niyyath and some of the them are not even having their
sehari. Is this fast acceptable to Allah (c.c)?
No, the fast is not valid without intention (Niyyah).
It was narrated by Ibn ‘Umar from Hafsah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not have the intention of fasting before Fajr, there is no fast for him.” Narrated by Imam Ahmad, the authors of al-Sunan, Ibn Khuzaymah and Ibn Hibbaan
However, the Niyyah does not need to be verbal, it is sufficient to have in in the heart. For example, if you make intention in the night that you are going to fast tomorrow, that is enough.
Prophet (peace and blessings of Allaah be upon him) said: “Actions are but by intentions, and everyone shall have but that which he intended…” (Narrated by al-Bukhaari, 1; Muslim, 1907)
Fast is valid without Suhoor (Sehari), but it is recommended to have Suhoor.
Shaykh Ibn Baaz (may Allaah have mercy on him) said: “Suhoor is not a condition for fasting to be valid, rather it is mustahabb (recommended), because the Prophet (peace and blessings of Allaah be upon him) said: “Have suhoor, for in suhoor there is blessing.” (Agreed upon).