https://www.trainerroad.com/forum/u/Captain_Doughnutman: your maths seems to be out.
2 months = 8 weeks, say = 48 sessions. 20% hard = 48/5=9.6.
8 weeks = 56 days. So hard sessions are once every 56/9.6 = 5.8 days.
Or in simple terms, every fifth session is hard. Most weeks, one hard session. every fifth week, two hard sessions.