1) If C2 is first day of Month,i.e, 10/01/2020:
In H3 then copy accross:
=IF(G3="",WORKDAY.INTL($C$2-1,1,"0111111"),WORKDAY(G3,1))
2) If C2 is kept as month in text, i.e, Oct, Nov ,... of the year 2020:
=IF(G3="",WORKDAY.INTL(DATEVALUE(1&$C$2&2020)-1,1,"0111111"),WORKDAY(G3,1))
Copy accross