Problem with getting date of first week of a year 2021

**Describe the bug**
I want to get date of a first week of 2021 (which is `2020-12-28`).
Dayjs gets `2019-12-30`. Momentjs gets correct date.
Here is my code:
```
dayjs('2021').day(1).week(1).toDate();
moment('2021').day(1).week(1).toDate();
```

**Expected behavior**
Should be `2020-12-28`

**Information**
- Day.js Version `1.10.4`
- Moment.js@2.27.0
- OS: `Windows 10`
- Browser `chrome 62`

Start by reproducing the provided Day.js 1.10.4 snippet and comparing its result with Moment.js. Trace the week calculation used by the .week(1) entry point, then verify that the result for 2021 is 2020-12-28.

Written by the indexing model from the issue text.