Our store is always crowded after 3 p.m. during the week and all weekend. Thank you for your understanding.
この場合、次のように書くと良いでしょう。
ーOur store is always crowded after 3 p.m. during the week and all weekend. Thank you for your understanding.
「当店は平日の午後3時以降と週末はいつも混雑しております。ご理解ください」
after 3 p.m. during the week「平日の午後3時以降」
all weekend「週末ずっと」
「平日の夕方」と言うなら、weekday evenings を使って、Our store is always crowded on weekday evenings ... のように言えますよ。
ご参考まで!