Rashi on Numbers 29:39/רש"י על במדבר כ״ט:ל״ט

1אלה תעשו לה׳ במועדיכם THESE YE SHALL OFFER TO THE LORD ON YOUR APPOINTED FESTIVALS — as something that is laid down as a duty,

2לבד מנדריכם BESIDES YOUR VOWS [AND YOUR FREEWILL OFFERINGS] — If on the Festival you wish to make a vow to bring a sacrifice, you have a duty ready at hand to your credit and you should bring it on that Festival; or in the case of vow-offerings and freewill offerings which you have vowed all the year, bring them on the Festival. — Perhaps it will be difficult for one to go up again to Jerusalem to bring his vow-offerings, and consequently he will transgress the command, (Deuteronomy 23:22) “Thou shalt not delay to pay it (thy vow)”.