"Friday is a day of the week."
Translation:El viernes es un día de la semana.
Try this, although it has exceptions (but few)...and for who thought this up, blame the Romans. It comes from Latin and all romance languages have it.
L-O-N-E-R-S: words that end naturally in these letters (not plurals) are usually masculine...notable exception "la mano"
D-IÓN-Z-A: words that end in these letters are usually feminine...notable exception: "el lápiz".
While not perfect, it's easier to remember the few exceptions to this than try to learn each word individually.
I get it, it just doesn't follow the general rule with words that end in "a", so it's annoying, that's all. It's kind of like the spelling rules about "ie" and "ei" in english, "i before e except after c" except sometimes that rule doesn't apply for no particular reason. Annoying as well.
There's really no surefire way of telling. I don't know when or how the gender of things was decided, but I doubt think it was based on any grammatical rule. Think of proper nouns. Most names that end with an 'a' are female, but we really just kind of know if a name is a girls' name or a boys' name through experience.
Most of the time, nouns follow the general masc/fem rule. Of course el dia and la radio are notable exceptions. Just remember that these are a couple odd ones out and you'll be fine