通过查看时间的.Weekday（），您可以计算第一个星期一。

package main

import (
    "fmt"
    "time"
)

// FirstMonday returns the day of the first Monday in the given month.
func FirstMonday(year int, month time.Month) int {
    t := time.Date(year, month, 1, 0, 0, 0, 0, time.UTC)
    return (8-int(t.Weekday()))%7 + 1
}

func main() {
    for m := 1; m <= 12; m++ {
        fmt.Println(m, FirstMonday(2013, time.Month(m)))
    }
}