It's literally Shellfish Day (blame me thinking the word clam sounds funny).
Pooossibly a menstruation euphemism? Possibly a non sequitur? Maybe the intent was that this chapter happened to take place on April 8th, which is actual Shellfish Day in Japan? I checked various sources but nothing made it make more sense. In any case the intent was that it was a hasty excuse by Suzurin and Tamao bought it. (Or did he?...)