A simple calendar view, some improvement from the original code: https://gist.github.com/mecid/f8859ea4bdbd02cf5d440d58e936faec/9169b0293f709bb1f560de2ca8184ea903fd5116

CalendarView.swift

//
//  CalendarView.swift
//  iWidget
//
//  Created by Huy Nguyen on 10/2/20.
//

import SwiftUI

fileprivate extension DateFormatter {
    static var month: DateFormatter {
        let formatter = DateFormatter()
        formatter.dateFormat = "MMMM"
        return formatter
    }

    static var monthAndYear: DateFormatter {
        let formatter = DateFormatter()
        formatter.dateFormat = "MMMM yyyy"
        return formatter
    }
}

fileprivate extension Calendar {
    func generateDates(
        inside interval: DateInterval,
        matching components: DateComponents
    ) -> [Date] {
        var dates: [Date] = []
        dates.append(interval.start)

        enumerateDates(
            startingAfter: interval.start,
            matching: components,
            matchingPolicy: .nextTime
        ) { date, _, stop in
            if let date = date {
                if date < interval.end {
                    dates.append(date)
                } else {
                    stop = true
                }
            }
        }

        return dates
    }
}

struct WeekView<DateView>: View where DateView: View {
    let week: Date
    let calendar: Calendar
    let highlightDate: Date?
    let content: (Date, Bool) -> DateView

    init(week: Date, calendar: Calendar, highlightDate: Date?, @ViewBuilder content: @escaping (Date, Bool) -> DateView) {
        self.week = week
        self.calendar = calendar
        self.highlightDate = highlightDate
        self.content = content
    }

    private var days: [Date] {
        guard
            let weekInterval = calendar.dateInterval(of: .weekOfYear, for: week)
            else { return [] }
        return calendar.generateDates(
            inside: weekInterval,
            matching: DateComponents(hour: 0, minute: 0, second: 0)
        )
    }

    var body: some View {
        HStack(spacing: 0) {
            ForEach(days, id: \.self) { date in
                if calendar.isDate(self.week, equalTo: date, toGranularity: .month) {
                    content(date, isHighlightDate(date))
                } else {
                   content(date, false)
                        .hidden()
                }
            }
        }
    }
    
    func isHighlightDate(_ date: Date) -> Bool {
        guard let highlightDate = highlightDate else { return false }
        return calendar.isDate(date, inSameDayAs: highlightDate)
    }
}

struct MonthView<DateView: View, WeekDate: View>: View {
    let month: Date
    let calendar: Calendar
    let weekDateView: (String) -> WeekDate
    let content: (Date, Bool) -> DateView
    let highlightDate: Date?

    init(month: Date, calendar: Calendar, highlightDate: Date?, @ViewBuilder weekDateView: @escaping (String) -> WeekDate, @ViewBuilder content: @escaping (Date, Bool) -> DateView) {
        self.month = month
        self.calendar = calendar
        self.highlightDate = highlightDate
        self.weekDateView = weekDateView
        self.content = content
    }
    
    init(month: Int, year: Int, calendar: Calendar, highlightDate: Date? = nil, @ViewBuilder weekDateView: @escaping (String) -> WeekDate, @ViewBuilder content: @escaping (Date, Bool) -> DateView) {
        var dateComponents = DateComponents()
        dateComponents.setValue(month, for: .month)
        dateComponents.setValue(year, for: .year)
        
        let month = Calendar.current.date(from: dateComponents) ?? Date()
        self.init(month: month, calendar: calendar, highlightDate: highlightDate, weekDateView: weekDateView, content: content)
    }

    private var weeks: [Date] {
        guard
            let monthInterval = calendar.dateInterval(of: .month, for: month)
            else { return [] }
        return calendar.generateDates(
            inside: monthInterval,
            matching: DateComponents(hour: 0, minute: 0, second: 0, weekday: calendar.firstWeekday)
        )
    }
    
    private var firstWeekDays: [Date] {
        guard let firstWeek = weeks.first,
            let weekInterval = calendar.dateInterval(of: .weekOfYear, for: firstWeek)
            else { return [] }
        return calendar.generateDates(
            inside: weekInterval,
            matching: DateComponents(hour: 0, minute: 0, second: 0)
        )
    }
    
    private let dateFormater = { () -> DateFormatter in
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "E"
        return dateFormatter
    }()

    var body: some View {
        VStack(spacing: 0) {
            HStack(spacing: 0) {
                ForEach(firstWeekDays, id: \.self) { date in
                    weekDateView(dateFormater.string(from: date))
                }
            }
            
            ForEach(weeks, id: \.self) { week in
                WeekView(week: week, calendar: calendar, highlightDate: highlightDate, content: self.content)
            }
        }
    }
}

struct CalendarView<DateView: View, WeekDate: View>: View {
    let interval: DateInterval
    let calendar: Calendar
    let highlightDate: Date?
    let weekDateView: (String) -> WeekDate
    let content: (Date, Bool) -> DateView

    init(interval: DateInterval, calendar: Calendar, highlightDate: Date?, @ViewBuilder weekDateView: @escaping (String) -> WeekDate, @ViewBuilder content: @escaping (Date, Bool) -> DateView) {
        self.interval = interval
        self.calendar = calendar
        self.highlightDate = highlightDate
        self.weekDateView = weekDateView
        self.content = content
    }

    private var months: [Date] {
        calendar.generateDates(
            inside: interval,
            matching: DateComponents(day: 1, hour: 0, minute: 0, second: 0)
        )
    }

    var body: some View {
        ScrollView(.vertical, showsIndicators: false) {
            VStack {
                ForEach(months, id: \.self) { month in
                    MonthView(month: month, calendar: calendar, highlightDate: highlightDate, weekDateView: weekDateView, content: self.content)
                }
            }
        }
    }
}

// Usage:

struct ContentView: View {
    let calendar: Calendar = { () -> Calendar in
        var calendar = Calendar.current
        calendar.firstWeekday = 0
        return calendar
    }()
    
    var body: some View {
        NavigationView {
            MonthView(month: 10, year: 2020, calendar: Calendar.current, highlightDate: Date(), weekDateView: { text in
                Text(text)
                    .font(.system(size: 13))
                    .frame(width: 30, height: 30)
                    .padding(.horizontal, 3)
            }) { date, isHighlight in
                Text(String(Calendar.current.component(.day, from: date)))
                    .foregroundColor(isHighlight ? Color.white : Color.orange)
                    .frame(width: 30, height: 30)
                    .background(isHighlight ? Color.orange : Color.clear)
                    .clipShape(Circle())
                    .padding(3)
            }
        }
    }
}