Hi, thanks for the feedback!

In general, Jewish Holidays begin the evening before the date specified. This is because the Jewish day actually begins at sundown on the previous night. Sometimes, for clarity, the Erev holiday is also included to indicate that the holiday begins the evening before.

We choose to add the Erev for major holidays, but don't typically add Erev for a minor holiday like Tu B'Av.

https://www.hebcal.com/home/47/when-do-the-holidays-begin

Hi, thanks for using Hebcal.

We don't celebrate Rosh Chodesh Tishrei on the first day of. Instead, we celebrate Rosh Hashana.

When the 9th of Av falls on Shabbat and the Tish’a B’Av fast is postponed to the 10th, the calendar event now says “Tish’a B’Av (observed)”.