There are a ton of things that can influence our day to day performance and heart rate. Sleep or lack there of, hydration, diet, etc. As well as exercise related muscular fatigue.
I can't tell you what has caused your off day.
But, most the published training plans will encourage one, "if in doubt, rest." You shouldn't need to get off the bike completely. But, just ease up for a few days. See how you feel after 4-7 days at a reduced work load. Then reapply yourself.
In answer to your direct question about how frequently "recovery" or "rest" weeks are normally scheduled: Depending on the nature of the plan, the intesnity of the workouts and the total accumulated fatigue, most plans will have a "recovery" week ever third or fourth week. Recovery weeks still include a fair bit of training, but, at reduced intensity and duration to allow muscles to repair actively after weeks of overload. Again, depending on the plan, "rest" weeks, which contain little to no training, may beschedule after 8-12 weeks, or not at all. It all depends.
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