fix: weekday check function for other locales

[cheeselemon]

Fixing issue #150 , #221

Problem:

• Calendar doesn't show previous month, all months start with 1.

Reason:

• This is due to getNumberOfDay being stuck at number = 0, when delcaring dayjs.locale() other than "en" globally outside library.

The Fix:

• Instead of using hard-coded weekday string array ["Sunday", ... "Saturday"], a function is created to specifically use previously set locales.