A working solution has been provided in another post:
Any way to pause at specific frames/time points with transition_reveal in gganimate?
airq_pause <- airq %>%
  mutate(show_time = case_when(Day %in% c(10,20,31) ~ 10,
                               TRUE                 ~ 1)) %>%
  # uncount is a tidyr function which copies each line 'n' times
  uncount(show_time) %>%
  group_by(Month) %>%
  mutate(reveal_time = row_number()) %>%
  ungroup()
Just edit the following line of code:
mutate(show_time = case_when(Day %in% c(10,20,31) ~ 10,
                                 TRUE             ~ 1),
to something along the lines of:
mutate(show_time = case_when(Day %in% max(Day) ~ 10,
                                 TRUE          ~ 1),
and adjust/remove the group_by() call accordingly.  It may be necessary to change the ~10 to a higher number to see the pause.