What happens if I forget to claim a daily reward?

If you forget to claim a reward on a given day, none of your current progress will be lost. 

You will simply be offered the same reward again on the next calendar day. 

Each calendar month lasts longer than the number of rewards available in the Nutaku Daily Reward Calendar. So even if you miss a few days, you should still be able to collect all the rewards available for that month!

Note, however, that the calendar rewards only progress whenever you claim a reward, so you will continue being offered the same daily reward until you claim it. There is no way to skip over particular rewards.