If it's just a 90-day report you can go anytime 2 weeks before or 1 week after. But try to avoid the week after Songkran when it will be extra busy. Mondays and Fridays are also best avoided.
Chang Puak bus station (just outside the north city wall) is for buses within the province, particularly the Chiang Dao, Fang, Thaton route. Anything outside the province leaves from Arcade.