Function isAveilut

isAveilut(date: number | HDate | Date): boolean
Returns true if the given date falls within a public period of communal mourning — specifically:
- Sefirat HaOmer: 16 Nisan through 5 Sivan (the 49 days of the Omer).
- Bein HaMetzarim ("between the straits"): 17 Tammuz through 9 Av (10 Av when 9 Av is postponed because it falls on Shabbat).
This is a broad helper — it does not attempt to model minhag-specific exceptions within those periods (e.g. Lag BaOmer, Rosh Chodesh Iyar, the distinction between Sephardic and Ashkenazic customs on which portion of the Omer is observed as mourning, etc.).
Parameters
- date: number | HDate | Date
  Hebrew Date, Gregorian date, or absolute R.D. day number
Returns boolean
true if the date is during a mourning period
Example
```
import {isAveilut, HDate, months} from '@hebcal/core';
isAveilut(new HDate(20, months.NISAN, 5784)); // true (Omer)
isAveilut(new HDate(25, months.TAMUZ, 5784)); // true (Three Weeks)
isAveilut(new HDate(15, months.AV, 5784));    // false
```
- Defined in src/isAveilut.ts:51