The ancient city is open every day except 25 December and 1 January. The first Sunday of each month is free for everyone at the gate. We do not sell that Sunday. The rest of the calendar is a question of weather and how long the gates stay open.
The long season
From 16 March to 14 October the city opens at 9:00, last entry is 17:30, and the gates close at 19:00. The day is capped at 20,000 entries. The morning holds 15,000 of them and the afternoon holds 5,000. Arrive near opening if you want the forum before it fills. Midday in July and August is hot, and most of the streets have no shade.
The short season
From 15 October to 15 March the city opens at 9:00, last entry is 15:30, and it closes at 17:00. The same daily cap applies. The walking is easier, and the day is shorter. Rain is more common. A wet morning does not close the city. It does make the stone slick.
The months that walk well
April, May, late September, and October are the moderate months: long enough hours, and less of the summer heat. June through August are the crowded, bright months. November through February are quieter and cooler, with a 15:30 last entry. These are patterns, not a forecast for your date.
Tuesday, and the other sites
Pompeii itself is open on Tuesday. Oplontis, the Stabiae villas, and the Stabiae museum are closed that day. If the note on your request asks for the three-day ticket, do not put a Tuesday against those sites. The city ticket on this form does not include them.
More to read

What to wear and bring to Pompeii
What to wear and bring to Pompeii: shoes with a tread, sun protection, a small bag, and the ID that matches the ticket.

Pompeii weather
Pompeii weather is bay weather: hot and open to the sun in summer, mild and wetter from autumn into early spring.
