how can I add MONTH+DAY to the X axis using ggplot2

Create a "date" class variable composed of your MONTH and DAY variables for plotting purposes, see this example with made-up data

library(tidyverse)
library(lubridate)
library(scales)

sample_df <- data.frame(
    month = rep(1:6, each = 30),
    day = rep(1:30, 6),
    so2 = rnorm(180)
)

sample_df %>% 
    mutate(date = ymd(paste("2021", month, day, sep = "-"))) %>% 
    drop_na() %>%
    ggplot(aes(x = date, y = so2)) +
    geom_point() +
    scale_x_date(date_breaks = "10 days",
                 labels = label_date_short(format = c(NA, "%B", "%d", "%H:%M")),
                 expand = c(0.005,0.005))
#> Warning: 2 failed to parse.

Created on 2021-05-17 by the reprex package (v2.0.0)

If you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.